Python Cheat Sheet

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Python Cheat Sheet

تاريخ النشر : Jan. 5, 2024

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المصدر الأصلي الماخوذ منه

https://programmingwithmosh.com/wp-content/uploads/2019/02/Python-Cheat-Sheet.pdf

Variables

We use variables to temporarily store data in computer’s memory.

price = 10
rating = 4.9 
course_name = ‘Python for Beginners’ 
is_published = True

In the above example,

• price is an integer (a whole number without a decimal point)

• rating is a float (a number with a decimal point)

• course_name is a string (a sequence of characters)

• is_published is a boolean. Boolean values can be True or False.

Comments

We use comments to add notes to our code. Good comments explain the hows and whys, not what the code does. That should be reflected in the code itself. Use comments to add reminders to yourself or other developers, or also explain your assumptions and the reasons you’ve written code in a certain way.

# This is a comment and it won’t get executed.  
# Our comments can be multiple lines.

Receiving Input

We can receive input from the user by calling the input() function.

birth_year = int(input(‘Birth year: ‘))

The input() function always returns data as a string. So, we’re converting the result into an integer by calling the built-in int() function.

Strings

We can define strings using single (' ') or double (" ") quotes. To define a multi-line string, we surround our string with tripe quotes (" " "). We can get individual characters in a string using square brackets [].

course = ‘Python for Beginners’ 
course[0]   # returns the first character 
course[1]   # returns the second character 
course[-1]  # returns the first character from the end 
course[-2]  # returns the second character from the end

We can slice a string using a similar notation:

course[1:5]

The above expression returns all the characters starting from the index position of 1 to 5 (but excluding 5).

The result will be ytho

If we leave out the start index, 0 will be assumed.

If we leave out the end index, the length of the string will be assumed.

We can use formatted strings to dynamically insert values into our strings:

name = ‘Mosh’ 
message = f’Hi, my name is {name}’ 
message.upper()   # to convert to uppercase 
message.lower()   # to convert to lowercase 
message.title()   # to capitalize the first letter of every word 
message.find(‘p’) # returns the index of the first occurrence of p  (or -1 if not found) 
message.replace(‘p’, ‘q’)

To check if a string contains a character (or a sequence of characters), we use the in operator:

contains = ‘Python’ in course

Arithmetic Operations

/ # returns a float

// # returns an int

% # returns the remainder of division

** # exponentiation - x ** y = x to the power of y

Augmented assignment operator:

x = x + 10 x += 10

Operator precedence:

1. parenthesis

2. exponentiation

3.multiplication / division

4. addition / subtraction

If Statements

if is_hot:
print(“hot day”)  elif is_cold:    print(“cold day”)  else:     print(“beautiful day”)

Logical operators:

if has_high_income and has_good_credit:
     ...  
if has_high_income or has_good_credit:
     ...  
is_day = True 
is_night = not is_day

Comparison operators

a > b
a >= b (greater than or equal to)  
a < b  a <= b  a == b (equals)  
a != b (not equals)

While loops

i = 1 
while i < 5:
    print(i)
    i += 1

For loops

for i in range(1, 5):
     print(i)

range(5): generates 0, 1, 2, 3, 4
range(1, 5): generates 1, 2, 3, 4
range(1, 5, 2): generates 1, 3

Lists

numbers = [1, 2, 3, 4, 5]  
numbers[0]   # returns the first item 
numbers[1]           # returns the second item 
numbers[-1]          # returns the first item from the end 
numbers[-2]          # returns the second item from the end 
numbers.append(6)    # adds 6 to the end 
numbers.insert(0, 6) # adds 6 at index position of 0 

numbers.remove(6)    # removes 6 
numbers.pop()        # removes the last item 
numbers.clear()      # removes all the items 
numbers.index(8)     # returns the index of first occurrence of 8 
numbers.sort()  # sorts the list 
numbers.reverse()    # reverses the list 
numbers.copy()       # returns a copy of the list

Tuples

They are like read-only lists. We use them to store a list of items. But once we define a tuple, we cannot add or remove items or change the existing items.

coordinates = (1, 2, 3)

We can unpack a list or a tuple into separate variables:

x, y, z = coordinates

Dictionaries

We use dictionaries to store key/value pairs.

customer = {    
“name”: “John Smith”,    
“age”: 30,    
“is_verified”: True 
}

We can use strings or numbers to define keys. They should be unique. We can use any types for the values.

customer[“name”]               # returns “John Smith” 
customer[“type”]               # throws an error 
customer.get(“type”, “silver”) # returns “silver” 
customer[“name”] = “new name”

Functions

We use functions to break up our code into small chunks. These chunks are easier to read, understand and maintain. If there are bugs, it’s easier to find bugs in a small chunk than the entire program. We can also re-use these chunks.

def greet_user(name):
      print(f”Hi {name}”)   

greet_user(“John”)

Parameters are placeholders for the data we can pass to functions. Arguments are the actual values we pass.

We have two types of arguments:

• Positional arguments: their position (order) matters

• Keyword arguments: position doesn’t matter - we prefix them with the parameter name.

# Two positional arguments greet_user(“John”, “Smith”) # Keyword arguments calculate_total(order=50, shipping=5, tax=0.1)

Our functions can return values. If we don’t use the return statement, by default None is returned. None is an object that represents the absence of a value.

def square(number):     return number * number   result = square(2)  print(result)  # prints 4

Exceptions Exceptions are errors that crash our programs. They often happen because of bad input or programming errors. It’s our job to anticipate and handle these exceptions to prevent our programs from cashing.

try:     age = int(input(‘Age: ‘))    income = 20000    risk = income / age     print(age)  except ValueError:    print(‘Not a valid number’)  except ZeroDivisionError:    print(‘Age cannot be 0’)

Classes We use classes to define new types.

class Point:     def __init__(self, x, y):         self.x = x        self.y = y     def move(self):        print(“move”)

When a function is part of a class, we refer to it as a method. Classes define templates or blueprints for creating objects. An object is an instance of a class. Every time we create a new instance, that instance follows the structure we define using the class. point1 = Point(10, 5)  point2 = Point(2, 4) __init__ is a special method called constructor. It gets called at the time of creating new objects. We use it to initialize our objects. Inheritance Inheritance is a technique to remove code duplication. We can create a base class to define the common methods and then have other classes inherit these methods.

class Mammal:    def walk(self):       print(“walk”)  class Dog(Mammal):    def bark(self):       print(“bark”)  dog = Dog()  dog.walk()   # inherited from Mammal dog.bark()   # defined in Dog

Modules A module is a file with some Python code. We use modules to break up our program into multiple files. This way, our code will be better organized. We won’t have one gigantic file with a million lines of code in it! There are 2 ways to import modules: we can import the entire module, or specific objects in a module.

# importing the entire converters module import converters  converters.kg_to_lbs(5) # importing one function in the converters module from converters import kg_to_lbs  kg_to_lbs(5)

Packages A package is a directory with __init__.py in it. It can contain one or more modules.

# importing the entire sales module from ecommerce import sales  sales.calc_shipping() # importing one function in the sales module from ecommerce.sales import calc_shipping  calc_shipping()

Python Standard Library Python comes with a huge library of modules for performing common tasks such as sending emails, working with date/time, generating random values, etc.

Random Module

import random   random.random()       # returns a float between 0 to 1 random.randint(1, 6)  # returns an int between 1 to 6 members = [‘John’, ‘Bob’, ‘Mary’]  leader = random.choice(members) # randomly picks an item

Pypi

Python Package Index (pypi.org) is a directory of Python packages published by Python developers around the world. We use pip to install or uninstall these packages.

pip install openpyxl 
pip uninstall openpyxl

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