Beginner`s Python Cheat Sheet: Variables and Strings, Lists

Lists (cont.)DictionariesBeginner's PythonDictionaries store connections between pieces ofList comprehensionsinformation. Each item in a dictionary is a key-value pair.Cheat Sheetsquares = [x**2 for x in range(1, 11)1A simple dictionarySlicing a listalien = {'color' 'green', 'points' 5}finishers = ['sam', 'bob', 'ada', 'bea'Variables and StringsAccessing a valuefirst_two = finishers[:2]Variables are used to store values. A string is a series ofprint ("The alien's color is alien['color'])Copying a listcharacters, surrounded by single or double quotes.Adding a new key-value paircopy_of_bikes = bikes[:]Hello worldalien['x_position'] = 0print ("Hello world!")TuplesLooping through all key-value pairsHello world with a variableTuples are similar to lists, but the items in a tuple can't beFav_numbers {'eric's 17, 'ever' : 4}modified.msg ="Hello world!"for name, number in fav_numbers.items() :print(msg)Making a tupleprint (name + loves + (number))Concatenation (combining strings)dimensions (1920, 1080)Looping through all keysfirst_name = 'albert'fav_numbers = {'eric': 17, 'ever': 4}last_name = 'einstein'If statementsfor name in fav_numbers.keys()full_name= first_name+ last_nameprint (name + loves a number')If statements are used to test for particular conditions andprint(full_name)respond appropriately.Looping through all the valuesConditional testsfav_numbers = {'eric': 17, 'ever': 4}Listsfor number in fav_numbers.values()A list stores a series of items in a particular order. Youequalsx == 42print(str(number) + is a favorite')not equalx != 42access items using an index, or within a loop.greater thanx > 42Make a listor equal tox >= 42User inputless thanx < 42bikes = ['trek', 'redline', 'giant']Your programs can prompt the user for input. All input isor equal tox << = 22stored as a string.Get the first item in a listConditional test with listsPrompting for a valuefirst_bike = bikes[0]'trek' in bikesname = input ("What's your name? ")Get the last item in a list'surly' not in bikesprint ("Hello, + name + "!")last_bike = bikes[-1]Assigning boolean valuesPrompting for numerical inputLooping through a listgame_active = Trueage = input ("How old are you? ")can_edit = Falsefor bike in bikes:age = (age)print(bike)A simple if testpi = input ("What's the value of pi? ")Adding items to a listif age >= 18:pi = float(pi)print("You can vote!")bikes = []bikes. append('trek')If-elif-else statementsbikes. append ('redline')if age < 4:bikes. append ('giant')ticket_price = 0Making numerical listselif age < 18:ticket_price = 10squares = []else:for X in range(1, 11)ticket_price = 15squares.append(x**2) While loopsClassesWorking with filesA while loop repeats a block of code as long as a certainA class defines the behavior of an object and the kind ofYour programs can read from files and write to files. Filescondition is true.information an object can store. The information in a classare opened in read mode ('r') by default, but can also beis stored in attributes, and functions that belong to a classopened in write mode ('w') and append mode ('a').A simple while loopare called methods. A child class inherits the attributes andcurrent_value = 1methods from its parent class.Reading a file and storing its lineswhile current_valuefilename = ssiddhartha.txt"Creating a dog classprint(current_value)with open(filename) as file_object:current_value += 1class Dog()lines = file_object.readlines()"""Represent dog.Letting the user choose when to quitfor line in lines:msg =def init (self, name)print(line)while msg!='quit':'""Initialize dog object."""msg = input ("What's your message? ")self.name = nameWriting to a fileprint(msg)filename = 'journal.txt'def sit(self)with open(filename, 'w') as file_object:Functions"""Simulate sitting."file_object.write("I love programming."print(self.name + is sitting.")Functions are named blocks of code, designed to do oneAppending to a filespecific job. Information passed to a function is called anmy_dog = Dog('Peso')argument, and information received by a function is called afilename 'journal.txt'with open(filename, 'a') as file_object:parameter.print(my_dog.name is a great dog!")file_object.write("\nI love making games.")A simple functionmy_dog.sit()def greet_user():InheritanceExceptions"""Display a simple greeting."""class SARDog(Dog)Exceptions help you respond appropriately to errors thatprint("Hello!")"""Represent a search dog." " "are likely to occur. You place code that might cause anerror in the try block. Code that should run in response togreet_user()def init (self, name)an error goes in the except block. Code that should run onlyPassing argument"""Initialize the sardog.""if the try block was successful goes in the else block.super()._init__(name)def greet_user(username):Catching an exception""Display a personalized greeting."""def search(self)prompt = "How many tickets do you need?print("Hello, username + "!")"""Simulate searching.""num_tickets = input(prompt) =print(self.name searching.")greet_user('jesse')try:my_dog= SARDog('Willie')Default values for parametersnum_tickets=int(num_tickets)except ValueError:def make_pizza(topping='bacon'):print(my_dog.name is a search dog.")print("Please try again.")"Make a single-topping pizza.my_dog.sit()else:int("Have topping + 11 pizza!")my_dog.search()print ("Your tickets are printing.")make_pizza()Infinite Skillsmake_pizza('pepperoni')Zen of PythonIf you had infinite programming skills, what would youSimple is better than complexReturning a valuebuild?If you have a choice between a simple and a complexdef add_numbers(x, y):As you're learning to program, it's helpful to thinksolution, and both work, use the simple solution. Your"""Add two numbers and return the sum." "about the real-world projects you'd like to create. It'scode will be easier to maintain, and it will be easierreturn x + ya good habit to keep an "ideas" notebook that youfor you and others to build on that code later on.can refer to whenever you want to start a new project.sum = add_numbers(3, 5)print (sum)If you haven't done so already, take a few minutesand describe three projects you'd like to create. Beginner's PythonAdding elementsSorting a listYou can add elements to the end of a list, or you can insertThe sort() method changes the order of a list permanently.them wherever you like in a list.The sorted() function returns a copy of the list, leaving theCheat Sheet - Listsoriginal list unchanged. You can sort the items in a list inAdding an element to the end of the listalphabetical order, or reverse alphabetical order. You canusers.append(*amy'also reverse the original order of the list. Keep in mind thatlowercase and uppercase letters may affect the sort order.What are lists?Starting with an empty listSorting a list permanentlyusers = []A list stores a series of items in a particular order.users. append('val'users. sort ()Lists allow you to store sets of information in oneusers. append('bob')Sorting a list permanently in reverse alphabeticalplace, whether you have just a few items or millionsusers.append('mia')orderof items. Lists are one of Python's most powerfulInserting elements at a particular positionfeatures readily accessible to new programmers, andusers. sort (reverse=True)they tie together many important concepts inusers.insert(0, 'joe')programming.users.insert(3, 'bea')Sorting a list temporarilyprint (sorted(users))Defining a listRemoving elementsprint (sorted(users, reverse=True)Use square brackets to define a list, and use commas toYou can remove elements by their position in a list, or byReversing the order of a listseparate individual items in the list. Use plural names forthe value of the item. If you remove an item by its value,users. reverse()lists, to make your code easier to read.Python removes only the first item that has that value.Making a listDeleting an element by its positionLooping through a listusers ['val', 'bob', 'mia', 'ron', 'ned'del users [-1]Lists can contain millions of items, so Python provides anefficient way to loop through all the items in a list. WhenRemoving an item by its valueyou set up a loop, Python pulls each item from the list oneAccessing elementsusers. remove ('mia')at a time and stores it in a temporary variable, which youIndividual elements in a list are accessed according to theirprovide a name for. This name should be the singularposition, called the index. The index of the first element isversion of the list name.0, the index of the second element is 1, and so forth.Popping elementsThe indented block of code makes up the body of theNegative indices refer to items at the end of the list. To getIf you want to work with an element that you're removingloop, where you can work with each individual item. Anya particular element, write the name of the list and then thefrom the list, you can "pop" the element. If you think of thelines that are not indented run after the loop is completed.index of the element in square brackets.list as a stack of items, pop() takes an item off the top of thestack. By default pop() returns the last element in the list,Printing all items in a listGetting the first elementbut you can also pop elements from any position in the list.for user in users:first_user users[0]Pop the last item from a listprint (user)Getting the second elementmost_recent_user= users.pop()Printing a message for each item, and a separatesecond_user = users[1]print(most_recent_user)message afterwardsGetting the last elementPop the first item in a listfor user in users:print ("Welcome, 11 + user + "!")newest_user = users[-1]first_user= users.pop(0)print(first_user)print ("Welcome, we're glad to see you all!")Modifying individual itemsOnce you've defined a list, you can change individualList lengthelements in the list. You do this by referring to the index ofThe len() function returns the number of items in a list.the item you want to modify.Find the length of a listChanging an elementnum_users = len(users)users[0] = 'valerie'print havestr (num_users) + " users.")users [-2] = 'ronald' The range() functionCopying a listTuplesYou can use the range() function to work with a set ofTo copy a list make a slice that starts at the first item andA tuple is like a list, except you can't change the values in anumbers efficiently. The range() function starts at 0 byends at the last item. If you try to copy a list without usingtuple once it's defined. Tuples are good for storingdefault, and stops one number below the number passed tothis approach, whatever you do to the copied list will affectinformation that shouldn't be changed throughout the life ofit. You can use the list() function to efficiently generate athe original list as well.a program. Tuples are designated by parentheses insteadlarge list of numbers.of square brackets. (You can overwrite an entire tuple, butMaking a copy of a listyou can't change the individual elements in a tuple.)Printing the numbers 0 to 1000finishers = ['kai', 'abe', 'ada', 'gus', 'zoe']Defining a tuplefor number in range(1001)copy_of_finishers = finishers[:]print (number)dimensions = (800, 600)Printing the numbers 1 to 1000List comprehensionsLooping through a tuplefor number in range(1, 1001)You can use a loop to generate a list based on a range offor dimension in dimensionsprint (number)numbers or on another list. This is a common operation, soprint (dimension)Python offers a more efficient way to do it. ListMaking a list of numbers from 1 to a millioncomprehensions may look complicated at first; if so, use theOverwriting a tuplefor loop approach until you're ready to start usingnumbers = ist(range(1, 1000001))dimensions = (800, 600)comprehensions.print (dimensions)To write a comprehension, define an expression for theSimple statisticsvalues you want to store in the list. Then write a for loop todimensions = (1200, 900)There are a number of simple statistics you can run on a listgenerate input values needed to make the list.containing numerical data.Using a loop to generate a list of square numbersVisualizing your codeFinding the minimum value in a listsquares =When you're first learning about data structures such asages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77]for X in range(1, 11)lists, it helps to visualize how Python is working with theyoungest = min(ages)square = x**2information in your program. pythontutor.com is a great toolsquares.append(square)for seeing how Python keeps track of the information in aFinding the maximum valueUsing a comprehension to generate a list of squarelist. Try running the following code on pythontutor.com, andages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77]numbersthen run your own code.oldest = max(ages)squares = [x**2 for X in range(1, 11)Build a list and print the items in the listFinding the sum of all valuesdogs = []Using a loop to convert a list of names to upper caseages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77]dogs. append('willie')total_years = sum(ages)names = ['kai', 'abe', 'ada', 'gus', 'zoe']dogs. append('hootz')dogs. append('peso')upper_names = []dogs .append('goblin')Slicing a listfor name in names:You can work with any set of elements from a list. A portionupper_names.append(name.upper())for dog in dogs:of a list is called a slice. To slice a list start with the index ofprint("Hello"+ dogthe first item you want, then add a colon and the index afterUsing a comprehension to convert a list of names toprint("I love these_dogs!")the last item you want. Leave off the first index to start atupper casethe beginning of the list, and leave off the last index to slicenames = ['kai', 'abe', 'ada', 'gus', 'zoe']print("\nThese were my first two dogs:"through the end of the list.old_dogs = dogs[:2]Getting the first three itemsupper_names = [name.upper() for name in names]for old_dog in old_dogs:print(old_dog)finishers = ['kai', 'abe', 'ada', 'gus', 'zoe']first_three = finishers :3]Styling your codedel dogs [0]Readability countsdogs.remove('peso')Getting the middle three itemsprint (dogs)Use four spaces per indentation level.middle_three = finishers[1:4)Keep your lines to 79 characters or fewer.Getting the last three itemsUse single blank lines to group parts of yourlast_three = finishers[-3:]program visually. Beginner's PythonAdding new key-value pairsLooping through a dictionaryYou can store as many key-value pairs as you want in aYou can loop through a dictionary in three ways: you candictionary, until your computer runs out of memory. To addloop through all the key-value pairs, all the keys, or all theCheat Sheet -a new key-value pair to an existing dictionary give the namevalues.of the dictionary and the new key in square brackets, andA dictionary only tracks the connections between keysset it equal to the new value.and values; it doesn't track the order of items in theThis also allows you to start with an empty dictionary anddictionary. If you want to process the information in order,Dictionariesadd key-value pairs as they become relevant.you can sort the keys in your loop.Adding a key-value pairLooping through all key-value pairsWhat are dictionaries?alien_0 {'color': 'green', 'points' 5}# Store people's favorite languages.fav_languagesPython's dictionaries allow you to connect pieces ofralien_0['x'] :jen' 'python',alien_0['y'] 25sarah' 'c',related information. Each piece of information in adictionary is stored as a key-value pair. When youalien_0[ speed'] = 1.5'edward' 'ruby',phil' 'python',provide a key, Python returns the value associatedAdding empty dictionary}with that key. You can loop through all the key-valuealien_0 { { }pairs, all the keys, or all the values.alien_o['color'l='green# Show each person's favorite language.alien_0['points']=5for name, language in fav_languages.items(print (name + ": + language)Defining a dictionaryUse curly braces to define a dictionary. Use colons toModifying valuesLooping through all the keysconnect keys and values, and use commas to separateYou can modify the value associated with any key in a# Show everyone who's taken the survey.individual key-value pairs.dictionary. To do so give the name of the dictionary andfor name in fav_languages. keys () :Making a dictionaryenclose the key in square brackets, then provide the newprint(name)value for that key.alien_0 = {'color' 'green', 'points': 5}Looping through all the valuesModifying values in a dictionary# Show all the languages that have been chosen.Accessing valuesalien_0 {'color': 'green', 'points': 5}for language in fav_languages.values()print(alien_0)To access the value associated with an individual key giveprint (language)the name of the dictionary and then place the key in a set of# Change the alien's color and point value.Looping through all the keys in ordersquare brackets. If the key you're asking for is not in thealien_0['color'] = 'yellow'dictionary, an error will occur.# Show each person's favorite language,alien_0['points'] = 10You can also use the get() method, which returns None# in order by the person's name.print(alien_0)instead of an error if the key doesn't exist. You can alsofor name in sorted(fav_languages.keys())specify a default value to use if the key is not in theprint (name + ": + language)dictionary.Removing key-value pairsGetting the value associated with a keyYou can remove any key-value pair you want from aDictionary lengthdictionary. To do so use the del keyword and the dictionaryYou can find the number of key-value pairs in a dictionary.alien_0 = {'color': 'green', 'points': 5}name, followed by the key in square brackets. This willdelete the key and its associated value.Finding a dictionary's lengthprint(alien_0['color'])print(alien_0['points'])Deleting a key-value pairnum_responses = len(fav_languages)Gettingthevalue with get()alien_0 = {'color': 'green', 'points': 5}print(alien_0)alien_0 ={'color':'green'}delalien_0['points']alien_color = alien_0.get('color')print(alien_0)alien_points = alien_0.get('points', 0)Visualizing dictionariesint(alien_color)print (alien_points)Try running some of these examples on pythontutor.com.