【无标题】 - 技术栈

|------------------------------ | Beginner's Python Cheat Sheet | Variables and Strings | Variables are used to | Hello world | print("Hello world!") | Hello world with a variable | msg = "Hello world!" print(msg) | f-strings (using variables in strings) | first_name = 'albert' | Lists | A list stores a series | Make a list | bikes = ['trek', 'redline', 'giant'] | Get the first item in a list | first_bike = bikes[0] | Get the last item in a list | last_bike = bikes[-1] | Looping through a list | for bike in bikes: print(bike) | Adding items to a list | bikes = [] bikes.append('trek') | Making numerical lists | squares = [] for x in -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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assign labels to values. A string is a series of characters, surrounded by single or double quotes. Python's f-strings allow you to use variables inside strings to build dynamic messages. |
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last_name = 'einstein' full_name = f"{first_name} {last_name}" print(full_name) |
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of items in a particular order. You access items using an index, or within a loop. |
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bikes.append('redline') bikes.append('giant') |
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range(1, 11): squares.append(x**2) |

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| Lists (cont.) |
| List comprehensions |
| squares = [x**2 for x in range(1, 11)] |
| Slicing a list |
| finishers = ['sam', 'bob', 'ada', 'bea'] first_two = finishers[:2] |
| Copying a list |
| copy_of_bikes = bikes[:] |
| Tuples |
| Tuples are similar to lists, but the items in a tuple can't be modified. |
| Making a tuple |
| dimensions = (1920, 1080) resolutions = ('720p', '1080p', '4K') |
| If statements |
| If statements are used to test for particular conditions and respond appropriately. |
| Conditional tests |
| equal x == 42 not equal x != 42 greater than x > 42 or equal to x >= 42 less than x < 42 or equal to x <= 42 |
| Conditional tests with lists |
| 'trek' in bikes 'surly' not in bikes |
| Assigning boolean values |
| game_active = True can_edit = False |
| A simple if test |
| if age >= 18: print("You can vote!") |
| If-elif-else statements |
| if age < 4: ticket_price = 0 elif age < 18: ticket_price = 10 elif age < 65: ticket_price = 40 else: ticket_price = 15 |

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| Dictionaries |
| Dictionaries store connections between pieces of information. Each item in a dictionary is a key-value pair. |
| A simple dictionary |
| alien = {'color': 'green', 'points': 5} |
| Accessing a value |
| print(f"The alien's color is {alien['color']}.") |
| Adding a new key-value pair |
| alien['x_position'] = 0 |
| Looping through all key-value pairs |
| fav_numbers = {'eric': 7, 'ever': 4, 'erin': 47} for name, number in fav_numbers.items(): print(f"{name} loves {number}.") |
| Looping through all keys |
| fav_numbers = {'eric': 7, 'ever': 4, 'erin': 47} for name in fav_numbers.keys(): print(f"{name} loves a number.") |
| Looping through all the values |
| fav_numbers = {'eric': 7, 'ever': 4, 'erin': 47} for number in fav_numbers.values(): print(f"{number} is a favorite.") |
| User input |
| Your programs can prompt the user for input. All input is stored as a string. |
| Prompting for a value |
| name = input("What's your name? ") print(f"Hello, {name}!") |
| Prompting for numerical input |
| age = input("How old are you? ") age = int(age) pi = input("What's the value of pi? ") pi = float(pi) |

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| Python Crash Course A Hands-on, Project-Based Introduction to Programming ehmatthes.github.io/pcc_3e |

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| While loops |
| A while loop repeats a block of code as long as a certain condition is true. While loops are especially useful when you can't know ahead of time how many times a loop should run. |
| A simple while loop |
| current_value = 1 while current_value <= 5: print(current_value) current_value += 1 |
| Letting the user choose when to quit |
| msg = '' while msg != 'quit': msg = input("What's your message? ") if msg != 'quit': print(msg) |
| Functions |
| Functions are named blocks of code, designed to do one specific job. Information passed to a function is called an argument, and information received by a function is called a parameter. |
| A simple function |
| def greet_user(): """Display a simple greeting.""" print("Hello!") greet_user() |
| Passing an argument |
| def greet_user(username): """Display a personalized greeting.""" print(f"Hello, {username}!") greet_user('jesse') |
| Default values for parameters |
| def make_pizza(topping='pineapple'): """Make a single-topping pizza.""" print(f"Have a {topping} pizza!") make_pizza() make_pizza('mushroom') |
| Returning a value |
| def add_numbers(x, y): """Add two numbers and return the sum.""" return x + y sum = add_numbers(3, 5) print(sum) |

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| Classes |
| A class defines the behavior of an object and the kind of information an object can store. The information in a class is stored in attributes, and functions that belong to a class are called methods. A child class inherits the attributes and methods from its parent class. |
| Creating a dog class |
| class Dog: """Represent a dog.""" def init(self, name): """Initialize dog object.""" self.name = name def sit(self): """Simulate sitting.""" print(f"{self.name} is sitting.") my_dog = Dog('Peso') print(f"{my_dog.name} is a great dog!") my_dog.sit() |
| Inheritance |
| class SARDog(Dog): """Represent a search dog.""" def init(self, name): """Initialize the sardog.""" super().init(name) def search(self): """Simulate searching.""" print(f"{self.name} is searching.") my_dog = SARDog('Willie') print(f"{my_dog.name} is a search dog.") my_dog.sit() my_dog.search() |
| Infinite Skills |
| If you had infinite programming skills, what would you build? |
| As you're learning to program, it's helpful to think about the real-world projects you'd like to create. It's a good habit to keep an "ideas" notebook that you can refer to whenever you want to start a new project. If you haven't done so already, take a few minutes and describe three projects you'd like to create. As you're learning you can write small programs that relate to these ideas, so you can get practice writing code relevant to topics you're interested in. |

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| Working with files |
| Your programs can read from files and write to files. The pathlib library makes it easier to work with files and directories. Once you have a path defined, you can work with the read_text() and write_text() methods. |
| Reading the contents of a file |
| The read_text() method reads in the entire contents of a file. You can then split the text into a list of individual lines, and then process each line as you need to. |
| from pathlib import Path path = Path('siddhartha.txt') contents = path.read_text() lines = contents.splitlines() for line in lines: print(line) |
| Writing to a file |
| path = Path('journal.txt') msg = "I love programming.") path.write_text(msg) |
| Exceptions |
| Exceptions help you respond appropriately to errors that are likely to occur. You place code that might cause an error in the try block. Code that should run in response to an error goes in the except block. Code that should run only if the try block was successful goes in the else block. |
| Catching an exception |
| prompt = "How many tickets do you need? " num_tickets = input(prompt) try: num_tickets = int(num_tickets) except ValueError: print("Please try again.") else: print("Your tickets are printing.") |
| Zen of Python |
| Simple is better than complex |
| If you have a choice between a simple and a complex solution, and both work, use the simple solution. Your code will be easier to maintain, and it will be easier for you and others to build on that code later on. |

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| Weekly posts about all things Python mostlypython.substack.com |

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| Beginner's Python Cheat Sheet - Lists |
| What are lists? |
| A list stores a series of items in a particular order. Lists allow you to store sets of information in one place, whether you have just a few items or millions of items. Lists are one of Python's most powerful features readily accessible to new programmers, and they tie together many important concepts in programming. |
| Defining a list |
| Use square brackets to define a list, and use commas to separate individual items in the list. Use plural names for lists, to make it clear that the variable represents more than one item. |
| Making a list |
| users = ['val', 'bob', 'mia', 'ron', 'ned'] |
| Accessing elements |
| Individual elements in a list are accessed according to their position, called the index. The index of the first element is 0, the index of the second element is 1, and so forth. Negative indices refer to items at the end of the list. To get a particular element, write the name of the list and then the index of the element in square brackets. |
| Getting the first element |
| first_user = users[0] |
| Getting the second element |
| second_user = users[1] |
| Getting the last element |
| newest_user = users[-1] |
| Modifying individual items |
| Once you've defined a list, you can change the value of individual elements in the list. You do this by referring to the index of the item you want to modify. |
| Changing an element |
| users[0] = 'valerie' users[1] = 'robert' users[-2] = 'ronald' |

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| Sorting a list |
| The sort() method changes the order of a list permanently. The sorted() function returns a copy of the list, leaving the original list unchanged. You can sort the items in a list in alphabetical order, or reverse alphabetical order. You can also reverse the original order of the list. Keep in mind that lowercase and uppercase letters may affect the sort order. |
| Sorting a list permanently |
| users.sort() |
| Sorting a list permanently in reverse alphabetical order |
| users.sort(reverse=True) |
| Sorting a list temporarily |
| print(sorted(users)) print(sorted(users, reverse=True)) |
| Reversing the order of a list |
| users.reverse() |
| Looping through a list |
| Lists can contain millions of items, so Python provides an efficient way to loop through all the items in a list. When you set up a loop, Python pulls each item from the list one at a time and assigns it to a temporary variable, which you provide a name for. This name should be the singular version of the list name. The indented block of code makes up the body of the loop, where you can work with each individual item. Any lines that are not indented run after the loop is completed. |
| Printing all items in a list |
| for user in users: print(user) |
| Printing a message for each item, and a separate message afterwards |
| for user in users: print(f"\nWelcome, {user}!") print("We're so glad you joined!") print("\nWelcome, we're glad to see you all!") |

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| Adding elements |
| You can add elements to the end of a list, or you can insert them wherever you like in a list. This allows you to modify existing lists, or start with an empty list and then add items to it as the program develops. |
| Adding an element to the end of the list |
| users.append('amy') |
| Starting with an empty list |
| users = [] users.append('amy') users.append('val') users.append('bob') users.append('mia') |
| Inserting elements at a particular position |
| users.insert(0, 'joe') users.insert(3, 'bea') |
| Removing elements |
| You can remove elements by their position in a list, or by the value of the item. If you remove an item by its value, Python removes only the first item that has that value. |
| Deleting an element by its position |
| del users[-1] |
| Removing an item by its value |
| users.remove('mia') |
| Popping elements |
| If you want to work with an element that you're removing from the list, you can "pop" the item. If you think of the list as a stack of items, pop() takes an item off the top of the stack. By default pop() returns the last element in the list, but you can also pop elements from any position in the list. |
| Pop the last item from a list |
| most_recent_user = users.pop() print(most_recent_user) |
| Pop the first item in a list |
| first_user = users.pop(0) print(first_user) |
| List length |
| The len() function returns the number of items in a list. |
| Find the length of a list |
| num_users = len(users) print(f"We have {num_users} users.") |

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| The range() function |
| You can use the range() function to work with a set of numbers efficiently. The range() function starts at 0 by default, and stops one number below the number passed to it. You can use the list() function to efficiently generate a large list of numbers. |
| Printing the numbers 0 to 1000 |
| for number in range(1001): print(number) |
| Printing the numbers 1 to 1000 |
| for number in range(1, 1001): print(number) |
| Making a list of numbers from 1 to a million |
| numbers = list(range(1, 1_000_001)) |
| Simple statistics |
| There are a number of simple statistical operations you can run on a list containing numerical data. |
| Finding the minimum value in a list |
| ages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77] youngest = min(ages) |
| Finding the maximum value |
| ages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77] oldest = max(ages) |
| Finding the sum of all values |
| ages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77] total_years = sum(ages) |
| Slicing a list |
| You can work with any subset of elements from a list. A portion of a list is called a slice. To slice a list start with the index of the first item you want, then add a colon and the index after the last item you want. Leave off the first index to start at the beginning of the list, and leave off the second index to slice through the end of the list. |
| Getting the first three items |
| finishers = ['kai', 'abe', 'ada', 'gus', 'zoe'] first_three = finishers[:3] |
| Getting the middle three items |
| middle_three = finishers[1:4] |
| Getting the last three items |
| last_three = finishers[-3:] |

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| Copying a list |
| To copy a list make a slice that starts at the first item and ends at the last item. If you try to copy a list without using this approach, whatever you do to the copied list will affect the original list as well. |
| Making a copy of a list |
| finishers = ['kai', 'abe', 'ada', 'gus', 'zoe'] copy_of_finishers = finishers[:] |
| List comprehensions |
| You can use a loop to generate a list based on a range of numbers or on another list. This is a common operation, so Python offers a more efficient way to do it. List comprehensions may look complicated at first; if so, use the for loop approach until you're ready to start using comprehensions. To write a comprehension, define an expression for the values you want to store in the list. Then write a for loop to generate input values needed to make the list. |
| Using a loop to generate a list of square numbers |
| squares = [] for x in range(1, 11): square = x**2 squares.append(square) |
| Using a comprehension to generate a list of square numbers |
| squares = [x**2 for x in range(1, 11)] |
| Using a loop to convert a list of names to upper case |
| names = ['kai', 'abe', 'ada', 'gus', 'zoe'] upper_names = [] for name in names: upper_names.append(name.upper()) |
| Using a comprehension to convert a list of names to upper case |
| names = ['kai', 'abe', 'ada', 'gus', 'zoe'] upper_names = [name.upper() for name in names] |
| Styling your code |
| Readability counts |
| Follow common Python formatting conventions: * Use four spaces per indentation level. * Keep your lines to 79 characters or fewer. * Use single blank lines to group parts of your program visually. |

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| Weekly posts about all things Python mostlypython.substack.com |

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| Beginner's Python Cheat Sheet - Dictionaries |
| What are dictionaries? |
| Python's dictionaries allow you to connect pieces of related information. Each piece of information in a dictionary is stored as a key-value pair. When you provide a key, Python returns the value associated with that key. You can loop through all the key-value pairs, all the keys, or all the values. |
| Defining a dictionary |
| Use curly braces to define a dictionary. Use colons to connect keys and values, and use commas to separate individual key-value pairs. |
| Making a dictionary |
| alien_0 = {'color': 'green', 'points': 5} |
| Accessing values |
| To access the value associated with an individual key give the name of the dictionary and then place the key in a set of square brackets. If the key you provided is not in the dictionary, an error will occur. You can also use the get() method, which returns None instead of an error if the key doesn't exist. You can also specify a default value to use if the key is not in the dictionary. |
| Getting the value associated with a key |
| alien_0 = {'color': 'green', 'points': 5} print(alien_0['color']) print(alien_0['points']) |
| Getting the value with get() |
| alien_0 = {'color': 'green'} alien_color = alien_0.get('color') alien_points = alien_0.get('points', 0) alien_speed = alien_0.get('speed') print(alien_color) print(alien_points) print(alien_speed) |

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| Adding new key-value pairs |
| You can store as many key-value pairs as you want in a dictionary, until your computer runs out of memory. To add a new key-value pair to an existing dictionary give the name of the dictionary and the new key in square brackets, and set it equal to the new value. This also allows you to start with an empty dictionary and add key-value pairs as they become relevant. |
| Adding a key-value pair |
| alien_0 = {'color': 'green', 'points': 5} alien_0['x'] = 0 alien_0['y'] = 25 alien_0['speed'] = 1.5 |
| Starting with an empty dictionary |
| alien_0 = {} alien_0['color'] = 'green' alien_0['points'] = 5 |
| Modifying values |
| You can modify the value associated with any key in a dictionary. To do so give the name of the dictionary and the key in square brackets, then provide the new value for that key. |
| Modifying values in a dictionary |
| alien_0 = {'color': 'green', 'points': 5} print(alien_0) # Change the alien's color and point value. alien_0['color'] = 'yellow' alien_0['points'] = 10 print(alien_0) |
| Removing key-value pairs |
| You can remove any key-value pair you want from a dictionary. To do this use the del keyword and the dictionary name, followed by the key in square brackets. This will delete the key and its associated value. |
| Deleting a key-value pair |
| alien_0 = {'color': 'green', 'points': 5} print(alien_0) del alien_0['points'] print(alien_0) |
| Visualizing dictionaries |
| Try running some of these examples on pythontutor.com, and then run one of your own programs that uses dictionaries. |

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| Nesting - Lists in a dictionary |
| Storing a list inside a dictionary allows you to associate more than one value with each key. |
| Storing lists in a dictionary |
| # Store multiple languages for each person. fav_languages = { 'jen': ['python', 'ruby'], 'sarah': ['c'], 'edward': ['ruby', 'go'], 'phil': ['python', 'haskell'], } # Show all responses for each person. for name, langs in fav_languages.items(): print(f"{name}: ") for lang in langs: print(f"- {lang}") |
| Nesting - A dictionary of dictionaries |
| You can store a dictionary inside another dictionary. In this case each value associated with a key is itself a dictionary. |
| Storing dictionaries in a dictionary |
| users = { 'aeinstein': { 'first': 'albert', 'last': 'einstein', 'location': 'princeton', }, 'mcurie': { 'first': 'marie', 'last': 'curie', 'location': 'paris', }, } for username, user_dict in users.items(): full_name = f"{user_dict['first']} " full_name += user_dict['last'] location = user_dict['location'] print(f"\nUsername: {username}") print(f"\tFull name: {full_name.title()}") print(f"\tLocation: {location.title()}") |
| Levels of nesting |
| Nesting is extremely useful in certain situations. However, be aware of making your code overly complex. If you're nesting items much deeper than what you see here there are probably simpler ways of managing your data, such as using classes. |

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| Dictionary Comprehensions |
| A comprehension is a compact way of generating a dictionary, similar to a list comprehension. To make a dictionary comprehension, define an expression for the key-value pairs you want to make. Then write a for loop to generate the values that will feed into this expression. The zip() function matches each item in one list to each item in a second list. It can be used to make a dictionary from two lists. |
| Using a loop to make a dictionary |
| squares = {} for x in range(5): squares[x] = x**2 |
| Using a dictionary comprehension |
| squares = {x:x**2 for x in range(5)} |
| Using zip() to make a dictionary |
| group_1 = ['kai', 'abe', 'ada', 'gus', 'zoe'] group_2 = ['jen', 'eva', 'dan', 'isa', 'meg'] pairings = {name:name_2 for name, name_2 in zip(group_1, group_2)} |
| Generating a million dictionaries |
| You can use a loop to generate a large number of dictionaries efficiently, if all the dictionaries start out with similar data. |
| A million aliens |
| aliens = [] # Make a million green aliens, worth 5 points # each. Have them all start in one row. for alien_num in range(1_000_000): new_alien = { 'color': 'green', 'points': 5, 'x': 20 * alien_num, 'y': 0 } aliens.append(new_alien) # Prove the list contains a million aliens. num_aliens = len(aliens) print("Number of aliens created:") print(num_aliens) |

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| Weekly posts about all things Python mostlypython.substack.com |

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| Beginner's Python Cheat Sheet - If Statements and While Loops |
| What are if statements? What are while loops? |
| Python's if statements allow you to examine the current state of a program and respond appropriately to that state. You can write a simple if statement that checks one condition, or you can create a complex series of statements that identify the exact conditions you're interested in. while loops run as long as certain conditions remain true. You can use while loops to let your programs run as long as your users want them to. |
| Conditional Tests |
| A conditional test is an expression that can be evaluated as true or false. Python uses the values True and False to decide whether the code in an if statement should be executed. |
| Checking for equality |
| A single equal sign assigns a value to a variable. A double equal sign checks whether two values are equal. If your conditional tests aren't doing what you expect them to, make sure you're not accidentally using a single equal sign. |
| >>> car = 'bmw' >>> car == 'bmw' True >>> car = 'audi' >>> car == 'bmw' False |
| Ignoring case when making a comparison |
| >>> car = 'Audi' >>> car.lower() == 'audi' True |
| Checking for inequality |
| >>> topping = 'mushrooms' >>> topping != 'anchovies' True |

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| Numerical comparisons |
| Testing numerical values is similar to testing string values. |
| Testing equality and inequality |
| >>> age = 18 >>> age == 18 True >>> age != 18 False |
| Comparison operators |
| >>> age = 19 >>> age < 21 True >>> age <= 21 True >>> age > 21 False >>> age >= 21 False |
| Checking multiple conditions |
| You can check multiple conditions at the same time. The and operator returns True if all the conditions listed are true. The or operator returns True if any condition is true. |
| Using and to check multiple conditions |
| >>> age_0 = 22 >>> age_1 = 18 >>> age_0 >= 21 and age_1 >= 21 False >>> age_1 = 23 >>> age_0 >= 21 and age_1 >= 21 True |
| Using or to check multiple conditions |
| >>> age_0 = 22 >>> age_1 = 18 >>> age_0 >= 21 or age_1 >= 21 True >>> age_0 = 18 >>> age_0 >= 21 or age_1 >= 21 False |
| Boolean values |
| A boolean value is either True or False. Variables with boolean values are often used to keep track of certain conditions within a program. |
| Simple boolean values |
| game_active = True is_valid = True can_edit = False |

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| If statements |
| Several kinds of if statements exist. Your choice of which to use depends on the number of conditions you need to test. You can have as many elif blocks as you need, and the else block is always optional. |
| Simple if statement |
| age = 19 if age >= 18: print("You're old enough to vote!") |
| If-else statements |
| age = 17 if age >= 18: print("You're old enough to vote!") else: print("You can't vote yet.") |
| The if-elif-else chain |
| age = 12 if age < 4: price = 0 elif age < 18: price = 25 else: price = 40 print(f"Your cost is ${price}.") |
| Conditional tests with lists |
| You can easily test whether a certain value is in a list. You can also test whether a list is empty before trying to loop through the list. |
| Testing if a value is in a list |
| >>> players = ['al', 'bea', 'cyn', 'dale'] >>> 'al' in players True >>> 'eric' in players False |
| Testing if two values are in a list |
| >>> 'al' in players and 'cyn' in players |

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| Conditional tests with lists (cont.) |
| Testing if a value is not in a list |
| banned_users = ['ann', 'chad', 'dee'] user = 'erin' if user not in banned_users: print("You can play!") |
| Checking if a list is empty |
| An empty list evaluates as False in an if statement. |
| players = [] if players: for player in players: print(f"Player: {player.title()}") else: print("We have no players yet!") |
| Accepting input |
| You can allow your users to enter input using the input() function. All input is initially stored as a string. If you want to accept numerical input, you'll need to convert the input string value to a numerical type. |
| Simple input |
| name = input("What's your name? ") print(f"Hello, {name}.") |
| Accepting numerical input using int() |
| age = input("How old are you? ") age = int(age) if age >= 18: print("\nYou can vote!") else: print("\nSorry, you can't vote yet.") |
| Accepting numerical input using float() |
| tip = input("How much do you want to tip? ") tip = float(tip) print(f"Tipped ${tip}.") |
| While loops |
| A while loop repeats a block of code as long as a condition is true. |
| Counting to 5 |
| current_number = 1 while current_number <= 5: print(current_number) current_number += 1 |

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| While loops (cont.) |
| Letting the user choose when to quit |
| prompt = "\nTell me something, and I'll " prompt += "repeat it back to you." prompt += "\nEnter 'quit' to end the program. " message = "" while message != 'quit': message = input(prompt) if message != 'quit': print(message) |
| Using a flag |
| Flags are most useful in long-running programs where code from other parts of the program might need to end the loop. |
| prompt = "\nTell me something, and I'll " prompt += "repeat it back to you." prompt += "\nEnter 'quit' to end the program. " active = True while active: message = input(prompt) if message == 'quit': active = False else: print(message) |
| Using break to exit a loop |
| prompt = "\nWhat cities have you visited?" prompt += "\nEnter 'quit' when you're done. " while True: city = input(prompt) if city == 'quit': break else: print(f"I've been to {city}!") |
| Accepting input with Sublime Text |
| Sublime Text, and a number of other text editors can't run programs that prompt the user for input. You can use these editors to write programs that prompt for input, but you'll need to run them from a terminal. |
| Breaking out of loops |
| You can use the break statement and the continue statement with any of Python's loops. For example you can use break to quit a for loop that's working through a list or a dictionary. You can use continue to skip over certain items when looping through a list or dictionary as well. |

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| Weekly posts about all things Python mostlypython.substack.com |

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| Beginner's Python Cheat Sheet - Functions |
| What are functions? |
| Functions are named blocks of code designed to do one specific job. Functions allow you to write code once that can then be run whenever you need to accomplish the same task. Functions can take in the information they need, and return the information they generate. Using functions effectively makes your programs easier to write, read, test, and maintain. |
| Defining a function |
| The first line of a function is its definition, marked by the keyword def. The name of the function is followed by a set of parentheses and a colon. A docstring, in triple quotes, describes what the function does. The body of a function is indented one level. To call a function, give the name of the function followed by a set of parentheses. |
| Making a function |
| def greet_user(): """Display a simple greeting.""" print("Hello!") greet_user() |
| Passing information to a function |
| Information that's passed to a function is called an argument; information that's received by a function is called a parameter. Arguments are included in parentheses after the function's name, and parameters are listed in parentheses in the function's definition. |
| Passing a simple argument |
| def greet_user(username): """Display a simple greeting.""" print(f"Hello, {username}!") greet_user('jesse') greet_user('diana') greet_user('brandon') |

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| Positional and keyword arguments |
| The two main kinds of arguments are positional and keyword arguments. When you use positional arguments Python matches the first argument in the function call with the first parameter in the function definition, and so forth. With keyword arguments, you specify which parameter each argument should be assigned to in the function call. When you use keyword arguments, the order of the arguments doesn't matter. |
| Using positional arguments |
| def describe_pet(animal, name): """Display information about a pet.""" print(f"\nI have a {animal}.") print(f"Its name is {name}.") describe_pet('hamster', 'harry') describe_pet('dog', 'willie') |
| Using keyword arguments |
| def describe_pet(animal, name): """Display information about a pet.""" print(f"\nI have a {animal}.") print(f"Its name is {name}.") describe_pet(animal='hamster', name='harry') describe_pet(name='willie', animal='dog') |
| Default values |
| You can provide a default value for a parameter. When function calls omit this argument the default value will be used. Parameters with default values must be listed after parameters without default values in the function's definition so positional arguments can still work correctly. |
| Using a default value |
| def describe_pet(name, animal='dog'): """Display information about a pet.""" print(f"\nI have a {animal}.") print(f"Its name is {name}.") describe_pet('harry', 'hamster') describe_pet('willie') |
| Using None to make an argument optional |
| def describe_pet(animal, name=None): """Display information about a pet.""" print(f"\nI have a {animal}.") if name: print(f"Its name is {name}.") describe_pet('hamster', 'harry') describe_pet('snake') |

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| Passing a list to a function |
| You can pass a list as an argument to a function, and the function can work with the values in the list. Any changes the function makes to the list will affect the original list. You can prevent a function from modifying a list by passing a copy of the list as an argument. |
| Passing a list as an argument |
| def greet_users(names): """Print a simple greeting to everyone.""" for name in names: msg = f"Hello, {name}!" print(msg) usernames = ['hannah', 'ty', 'margot'] greet_users(usernames) |
| Allowing a function to modify a list |
| The following example sends a list of models to a function for printing. The first list is emptied, and the second list is filled. |
| def print_models(unprinted, printed): """3d print a set of models.""" while unprinted: current_model = unprinted.pop() print(f"Printing {current_model}") printed.append(current_model) # Store some unprinted designs, # and print each of them. unprinted = ['phone case', 'pendant', 'ring'] printed = [] print_models(unprinted, printed) print(f"\nUnprinted: {unprinted}") print(f"Printed: {printed}") |
| Preventing a function from modifying a list |
| The following example is the same as the previous one, except the original list is unchanged after calling print_models(). |
| def print_models(unprinted, printed): """3d print a set of models.""" while unprinted: current_model = unprinted.pop() print(f"Printing {current_model}") printed.append(current_model) # Store some unprinted designs, # and print each of them. original = ['phone case', 'pendant', 'ring'] printed = [] print_models(original[:], printed) print(f"\nOriginal: {original}") print(f"Printed: {printed}") |

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| Modules |
| You can store your functions in a separate file called a module, and then import the functions you need into the file containing your main program. This allows for cleaner program files. Make sure your module is stored in the same directory as your main program. |
| Storing a function in a module |
| File: pizza.py |
| def make_pizza(size, *toppings): """Make a pizza.""" print(f"\nMaking a {size} pizza.") print("Toppings:") for topping in toppings: print(f"- {topping}") |
| Importing an entire module |
| File: making_pizzas.py Every function in the module is available in the program file. |
| import pizza pizza.make_pizza('medium', 'pepperoni') pizza.make_pizza('small', 'bacon', 'pineapple') |
| Importing a specific function |
| Only the imported functions are available in the program file. |
| from pizza import make_pizza make_pizza('medium', 'pepperoni') make_pizza('small', 'bacon', 'pineapple') |
| Giving a module an alias |
| import pizza as p p.make_pizza('medium', 'pepperoni') p.make_pizza('small', 'bacon', 'pineapple') |
| Giving a function an alias |
| from pizza import make_pizza as mp mp('medium', 'pepperoni') mp('small', 'bacon', 'pineapple') |
| Importing all functions from a module |
| Don't do this, but recognize it when you see it in others' code. It can result in naming conflicts, which can cause errors. |
| from pizza import * make_pizza('medium', 'pepperoni') make_pizza('small', 'bacon', 'pineapple') |

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| Passing an arbitrary number of arguments |
| Sometimes you won't know how many arguments a function will need to accept. Python allows you to collect an arbitrary number of arguments into one parameter using the * operator. A parameter that accepts an arbitrary number of arguments must come last in the function definition. This parameter is often named *args. The ** operator allows a parameter to collect an arbitrary number of keyword arguments. These are stored as a dictionary with the parameter names as keys, and the arguments as values. This is often named **kwargs. |
| Collecting an arbitrary number of arguments |
| def make_pizza(size, *toppings): """Make a pizza.""" print(f"\nMaking a {size} pizza.") print("Toppings:") for topping in toppings: print(f"- {topping}") # Make three pizzas with different toppings. make_pizza('small', 'pepperoni') make_pizza('large', 'bacon bits', 'pineapple') make_pizza('medium', 'mushrooms', 'peppers', 'onions', 'extra cheese') |
| Collecting an arbitrary number of keyword arguments |
| def build_profile(first, last, **user_info): """Build a dictionary for a user.""" user_info['first'] = first user_info['last'] = last return user_info # Create two users with different kinds # of information. user_0 = build_profile('albert', 'einstein', location='princeton') user_1 = build_profile('marie', 'curie', location='paris', field='chemistry') print(user_0) print(user_1) |
| What's the best way to structure a function? |
| There are many ways to write and call a function. When you're starting out, aim for something that simply works. As you gain experience you'll develop an understanding of the subtle advantages of different structures such as positional and keyword arguments, and the various approaches to importing functions. For now if your functions do what you need them to, you're doing well. |

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| Beginner's Python Cheat Sheet - Classes |
| |
| What are classes? |
| Classes are the foundation of object-oriented programming. Classes represent real-world things you want to model in your programs such as dogs, cars, and robots. You use a class to make objects, which are specific instances of dogs, cars, and robots. A class defines the general behavior that a whole category of objects can have, and the information that can be associated with those objects. Classes can inherit from each other---you can write a class that extends the functionality of an existing class. This allows you to code efficiently for a wide variety of situations. Even if you don't write many of your own classes, you'll frequently find yourself working with classes that others have written. |
| |
| Creating and using a class |
| Consider how we might model a car. What information would we associate with a car, and what behavior would it have? The information is assigned to variables called attributes, and the behavior is represented by functions. Functions that are part of a class are called methods. |
| The Car class |
| class Car: """A simple attempt to model a car.""" def init(self, make, model, year): """Initialize car attributes.""" self.make = make self.model = model self.year = year # Fuel capacity and level in gallons. self.fuel_capacity = 15 self.fuel_level = 0 def fill_tank(self): """Fill gas tank to capacity.""" self.fuel_level = self.fuel_capacity print("Fuel tank is full.") def drive(self): """Simulate driving.""" print("The car is moving.") |

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| Creating and using a class (cont.) |
| Creating an instance from a class |
| my_car = Car('audi', 'a4', 2021) |
| Accessing attribute values |
| print(my_car.make) print(my_car.model) print(my_car.year) |
| Calling methods |
| my_car.fill_tank() my_car.drive() |
| Creating multiple instances |
| my_car = Car('audi', 'a4', 2024) my_old_car = Car('subaru', 'outback', 2018) my_truck = Car('toyota', 'tacoma', 2020) my_old_truck = Car('ford', 'ranger', 1999) |
| |
| Modifying attributes |
| You can modify an attribute's value directly, or you can write methods that manage updating values more carefully. Methods like these can help validate the kinds of changes that are being made to an attribute. |
| Modifying an attribute directly |
| my_new_car = Car('audi', 'a4', 2024) my_new_car.fuel_level = 5 |
| Writing a method to update an attribute's value |
| def update_fuel_level(self, new_level): """Update the fuel level.""" if new_level <= self.fuel_capacity: self.fuel_level = new_level else: print("The tank can't hold that much!") |
| Writing a method to increment an attribute's value |
| def add_fuel(self, amount): """Add fuel to the tank.""" if (self.fuel_level + amount <= self.fuel_capacity): self.fuel_level += amount print("Added fuel.") else: print("The tank won't hold that much.") |
| Naming conventions |
| In Python class names are usually written in CamelCase and object names are written in lowercase with underscores. Modules that contain classes should be named in lowercase with underscores. |

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| Class inheritance |
| If the class you're writing is a specialized version of another class, you can use inheritance. When one class inherits from another, it automatically takes on all the attributes and methods of the parent class. The child class is free to introduce new attributes and methods, and override attributes and methods of the parent class. To inherit from another class include the name of the parent class in parentheses when defining the new class. |
| The init() method for a child class |
| class ElectricCar(Car): """A simple model of an electric car.""" def init(self, make, model, year): """Initialize an electric car.""" super().init(make, model, year) # Attributes specific to electric cars. # Battery capacity in kWh. self.battery_size = 40 # Charge level in %. self.charge_level = 0 |
| Adding new methods to the child class |
| class ElectricCar(Car): --snip-- def charge(self): """Fully charge the vehicle.""" self.charge_level = 100 print("The vehicle is fully charged.") |
| Using child methods and parent methods |
| my_ecar = ElectricCar('nissan', 'leaf', 2024) my_ecar.charge() my_ecar.drive() |
| Finding your workflow |
| There are many ways to model real world objects and situations in code, and sometimes that variety can feel overwhelming. Pick an approach and try it; if your first attempt doesn't work, try a different approach. |

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| Storing objects in a list |
| A list can hold as many items as you want, so you can make a large number of objects from a class and store them in a list. Here's an example showing how to make a fleet of rental cars, and make sure all the cars are ready to drive. |
| A fleet of rental cars |
| from car import Car, ElectricCar # Make lists to hold a fleet of cars. gas_fleet = [] electric_fleet = [] # Make 250 gas cars and 500 electric cars. for _ in range(250): car = Car('ford', 'escape', 2024) gas_fleet.append(car) for _ in range(500): ecar = ElectricCar('nissan', 'leaf', 2024) electric_fleet.append(ecar) # Fill the gas cars, and charge electric cars. for car in gas_fleet: car.fill_tank() for ecar in electric_fleet: ecar.charge() print(f"Gas cars: {len(gas_fleet)}") print(f"Electric cars: {len(electric_fleet)}") |
| |
| Understanding self |
| People often ask what the self variable represents. The self variable is a reference to an object that's been created from the class. The self variable provides a way to make other variables and objects available everywhere in a class. The self variable is automatically passed to each method that's called through an object, which is why you see it listed first in most method definitions. Any variable attached to self is available everywhere in the class. |
| |
| Understanding init() |
| The init() method is a function that's part of a class, just like any other method. The only special thing about init () is that it's called automatically every time you make a new instance from a class. If you accidentally misspell init(), the method won't be called and your object may not be created correctly. |

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| Beginner's Python Cheat Sheet - Files and Exceptions |
| Why work with files? Why use exceptions? |
| Your programs can read information in from files, and they can write data to files. Reading from files allows you to work with a wide variety of information; writing to files allows users to pick up where they left off the next time they run your program. You can write text to files, and you can store Python structures such as lists in data files as well. Exceptions are special objects that help your programs respond to errors in appropriate ways. For example if your program tries to open a file that doesn't exist, you can use exceptions to display an informative error message instead of having the program crash. |
| Reading from a file |
| To read from a file your program needs to specify the path to the file, and then read the contents of the file. The read_text() method returns a string containing the entire contents of the file. |
| Reading an entire file at once |
| from pathlib import Path path = Path('siddhartha.txt') contents = path.read_text() print(contents) |
| Working with a file's lines |
| It's often useful to work with individual lines from a file. Once the contents of a file have been read, you can get the lines using the splitlines() method. |
| from pathlib import Path path = Path('siddhartha.txt') contents = path.read_text() lines = contents.splitlines() for line in lines: print(line) |

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| Writing to a file |
| The write_text() method can be used to write text to a file. Be careful, this will write over the current file if it already exists. To append to a file, read the contents first and then rewrite the entire file. |
| Writing to a file |
| from pathlib import Path path = Path("programming.txt") msg = "I love programming!" path.write_text(msg) |
| Writing multiple lines to a file |
| from pathlib import Path path = Path("programming.txt") msg = "I love programming!" msg += "\nI love making games." path.write_text(msg) |
| Appending to a file |
| from pathlib import Path path = Path("programming.txt") contents = path.read_text() contents += "\nI love programming!" contents += "\nI love making games." path.write_text(contents) |
| Path objects |
| The pathlib module makes it easier to work with files in Python. A Path object represents a file or directory, and lets you carry out common directory and file operations. With a relative path, Python usually looks for a location relative to the .py file that's running. Absolute paths are relative to your system's root folder ("/"). Windows uses backslashes when displaying file paths, but you should use forward slashes in your Python code. |
| Relative path |
| path = Path("text_files/alice.txt") |
| Absolute path |
| path = Path("/Users/eric/text_files/alice.txt") |
| Get just the filename from a path |
| >>> path = Path("text_files/alice.txt") >>> path.name 'alice.txt' |

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| The else block |
| The try block should only contain code that may cause an error. Any code that depends on the try block running successfully should be placed in the else block. |
| Using an else block |
| print("Enter two numbers. I'll divide them.") x = input("First number: ") y = input("Second number: ") try: result = int(x) / int(y) except ZeroDivisionError: print("You can't divide by zero!") else: print(result) |
| Preventing crashes caused by user input |
| Without the except block in the following example, the program would crash if the user tries to divide by zero. As written, it will handle the error gracefully and keep running. |
| """A simple calculator for division only.""" print("Enter two numbers. I'll divide them.") print("Enter 'q' to quit.") while True: x = input("\nFirst number: ") if x == 'q': break y = input("Second number: ") if y == 'q': break try: result = int(x) / int(y) except ZeroDivisionError: print("You can't divide by zero!") else: print(result) |
| Deciding which errors to report |
| Well-written, properly tested code is not very prone to internal errors such as syntax or logical errors. But every time your program depends on something external such as user input or the existence of a file, there's a possibility of an exception being raised. It's up to you how to communicate errors to your users. Sometimes users need to know if a file is missing; sometimes it's better to handle the error silently. A little experience will help you know how much to report. |

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| Failing silently |
| Sometimes you want your program to just continue running when it encounters an error, without reporting the error to the user. Using the pass statement in an except block allows you to do this. |
| Using the pass statement in an except block |
| from pathlib import Path f_names = ['alice.txt', 'siddhartha.txt', 'moby_dick.txt', 'little_women.txt'] for f_name in f_names: # Report the length of each file found. path = Path(f_name) try: contents = path.read_text() except FileNotFoundError: pass else: lines = contents.splitlines() msg = f"{f_name} has {len(lines)}" msg += " lines." print(msg) |
| Avoid bare except blocks |
| Exception handling code should catch specific exceptions that you expect to happen during your program's execution. A bare except block will catch all exceptions, including keyboard interrupts and system exits you might need when forcing a program to close. If you want to use a try block and you're not sure which exception to catch, use Exception. It will catch most exceptions, but still allow you to interrupt programs intentionally. |
| Don't use bare except blocks |
| try: # Do something except: pass |
| Use Exception instead |
| try: # Do something except Exception: pass |
| Printing the exception |
| try: # Do something except Exception as e: print(e, type(e)) |

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| Storing data with json |
| The json module allows you to dump simple Python data structures into a file, and load the data from that file the next time the program runs. The JSON data format is not specific to Python, so you can share this kind of data with people who work in other languages as well. Knowing how to manage exceptions is important when working with stored data. You'll usually want to make sure the data you're trying to load exists before working with it. |
| Using json.dumps() to store data |
| from pathlib import Path import json numbers = [2, 3, 5, 7, 11, 13] path = Path("numbers.json") contents = json.dumps(numbers) path.write_text(contents) |
| Using json.loads() to read data |
| from pathlib import Path import json path = Path("numbers.json") contents = path.read_text() numbers = json.loads(contents) print(numbers) |
| Making sure the stored data exists |
| from pathlib import Path import json path = Path("numbers.json") try: contents = path.read_text() except FileNotFoundError: msg = f"Can't find file: {path}" print(msg) else: numbers = json.loads(contents) print(numbers) |
| Practice with exceptions |
| Take a program you've already written that prompts for user input, and add some error-handling code to the program. Run your program with appropriate and inappropriate data, and make sure it handles each situation correctly. |

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| Beginner's Python Cheat Sheet - Testing Your Code |
| Why test your code? |
| When you write a function or a class, you can also write tests for that code. Testing proves that your code works as it's supposed to in the situations it's designed to handle, and also when people use your programs in unexpected ways. Writing tests gives you confidence that your code will work correctly as more people begin to use your programs. You can also add new features to your programs and know whether or not you've broken existing behavior by running your tests. A unit test verifies that one specific aspect of your code works as it's supposed to. A test caseis a collection of unit tests which verify that your code's behavior is correct in a wide variety of situations. The output in some sections has been trimmed for space. |
| Testing a function: a passing test |
| The pytest library provides tools for testing your code. To try it out, we'll create a function that returns a full name. We'll use the function in a regular program, and then build a test case for the function. |
| A function to test |
| Save this as full_names.py |
| def get_full_name(first, last): """Return a full name.""" full_name = f"{first} {last}" return full_name.title() |
| Using the function |
| Save this as names.py |
| from full_names import get_full_name janis = get_full_name('janis', 'joplin') print(janis) bob = get_full_name('bob', 'dylan') print(bob) |

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| Installing pytest |
| Installing pytest with pip |
| $python -m pip install --user pytest | | Testing a function (cont.) | | Building a testcase with one unit test | | To build a test case, import the function you want to test. Any functions that begin with test_ will be run by pytest. Save this file as test_full_names.py. | | from full_names import get_full_name def test_first_last(): """Test names like Janis Joplin.""" full_name = get_full_name('janis', 'joplin') assert full_name == 'Janis Joplin' | | Running the test | | Issuing the pytest command tells pytest to run any file beginning with test_. pytest reports on each test in the test case. The dot after test_full_names.py represents a single passing test. pytest informs us that it ran 1 test in about 0.01 seconds, and that the test passed. | |$ pytest ============= test session starts ============= platform darwin -- Python 3.11.0, pytest-7.1.2 rootdir: /.../testing_your_code collected 1 item test_full_names.py . [100%] ============== 1 passed in 0.01s ============== |
| Testing a function: A failing test |
| Failing tests are important; they tell you that a change in the code has affected existing behavior. When a test fails, you need to modify the code so the existing behavior still works. |
| Modifying the function |
| We'll modify get_full_name() so it handles middle names, but we'll do it in a way that breaks existing behavior. |
| def get_full_name(first, middle, last): """Return a full name.""" full_name = f"{first} {middle} {last}" return full_name.title() |
| Using the function |
| from full_names import get_full_name john = get_full_name('john', 'lee', 'hooker') print(john) david = get_full_name('david', 'lee', 'roth') print(david) |

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| A failing test (cont.) |
| Running the test |
| When you change your code, it's important to run your existing tests. This will tell you whether the changes you made affect existing behavior. |
| $ pytest ============= test session starts ============= test_full_names_failing.py F [100%] ================== FAILURES =================== |