Python cheatsheet

• Python (python.org)
• Python Documentation (docs.python.org)
• Python has a large and active community of developers.

#Python is a

• high-level programming language, This makes it easy to learn and use.
• object-oriented programming language
• dynamically typed language, which means that the type of a variable is not checked at compile time.
• interpreted language, which means that it is not compiled into machine code before it is executed. This makes it very portable and easy to debug
• free and open source language,

full_name = "John Doe"
age = 25
married = False

• Python is a dynamically typed language, so you don't need to specify the type of the variable when you declare it. The type is inferred from the value you assign to it.
• Variables are case sensitive, so age and Age are two different variables.
• Variable names can only contain letters, numbers and underscores. They can't start with a number.
• Variable names should be descriptive, so you can understand what they are used for.
• Variable names should be in snake_case format, so each word is separated by an underscore.

• Single-line comments in Python start with the hash symbol (#).

#single line comment.

• Multi-line comments can be created by adding a multiline string (""" or ''') before and after the comment.

""" Multiline comment
"""

''' Multiline comment 2
'''

• Comments should be written in plain English (or the language of the code's audience) and follow standard conventions for spelling, grammar, and punctuation.

• Code should be organized into logical blocks with descriptive names and headers. Comments can be used to introduce and summarize these blocks.

• Comments can also be used to temporarily disable lines of code during testing or debugging. However, this should be used sparingly and documented clearly.

• A string is a sequence of one or more characters (letters, numbers, symbols) that can be either a constant or a variable.

string_1 = "string with double quotes"
string_2 = 'string with single quotes'

multi_string_1 = """Multiline Strings
can be written like this by 
wrapping inside three double quotes"""

multi_string_2 = '''Multiline Strings
can be written like this by
wrapping inside three single quotes'''

• You can access individual characters in a string using indexing.

name = "John Doe"
print(name[0]) # J
print(name[1]) # o
print(name[-1]) # e
print(name[-2]) # o

• You can access a substring from a string using slicing.

name = "John Doe"
print(name[1:4]) # ohn
print(name[:4]) # John
print(name[5:]) # Doe
print(name[1:7:2]) # oh o
print(name[::-1]) # eoD nhoJ

• You can use escape sequences to insert special characters in strings.

print("Hello\nWorld!") # Hello
                        # World!
print("Hello\tWorld!") # Hello   World!
print("\"Hello World!\"") # "Hello World!"
print("Hello\\World!") # Hello\World!

• Python has three numeric types: int, float, and complex.

age = 25    # int
radius = 8.3  # float
rec = 8j   # complex

• You can convert between numeric types using the int(), float(), and complex() methods.

age = 25    
print(float(age)) # 25.0
print(complex(age)) # (25+0j)

• You can add two numbers using the + operator.

x = 5
y = 3
print(x + y) # 8

• You can subtract two numbers using the - operator.

x = 5
y = 3
print(x - y) # 2

• You can multiply two numbers using the * operator.

x = 5
y = 3
print(x * y) # 15

• You can divide two numbers using the / operator.

x = 5
y = 3
print(x / y) # 1.6666666666666667

• You can get the remainder of two numbers using the % operator.

x = 5
y = 3
print(x % y) # 2

• You can raise a number to the power of another number using the ** operator.

x = 5
y = 3
print(x ** y) # 125

• You can get the integer part of a division using the // operator.

x = 5
y = 3
print(x // y) # 1

• Python has a built-in module called math, which extends the list of mathematical functions.

import math
print(math.pi) # 3.141592653589793
print(math.sqrt(25)) # 5.0
print(math.pow(2, 3)) # 8.0
print(math.floor(2.9)) # 2
print(math.ceil(2.1)) # 3

• Python has a built-in module called random, which can be used to make random numbers.

import random
print(random.randrange(1, 10)) # 7
print(random.random()) # 0.4283688941880216
print(random.choice([1, 2, 3, 4])) # 3

• You can round a number using the round() method.

print(round(2.1)) # 2
print(round(2.9)) # 3

• You can get the absolute value of a number using the abs() method.

print(abs(-2.9)) # 2.9

• You can create a complex number using the complex() method.

print(complex(2, 3)) # (2+3j)

• You can get the largest number in a list using the max() method.

print(max([1, 2, 3, 4])) # 4

• You can get the smallest number in a list using the min() method.

print(min([1, 2, 3, 4])) # 1

• You can get the sum of all numbers in a list using the sum() method.

print(sum([1, 2, 3, 4])) # 10

• Booleans represent one of two values: True or False.

x = True
y = False

• You can use the is keyword to check if two variables are the same object.

x = ["apple", "banana", "cherry"]
y = ["apple", "banana", "cherry"]
z = x
print(x is z) # True
print(x is y) # False
print(x == y) # True

• You can use the not keyword to invert a boolean.

print(not True) # False
print(not False) # True

• You can use the and keyword to combine two booleans.

print(True and True) # True
print(True and False) # False
print(False and True) # False
print(False and False) # False

• You can use the or keyword to combine two booleans.

print(True or True) # True
print(True or False) # True
print(False or True) # True
print(False or False) # False

• Lists are used to store multiple items in a single variable.

fruits = ["apple", "banana", "cherry"]

• You can also use the list() method to create a list.

fruits = list(("apple", "banana", "cherry"))

• You can access items in a list by referring to the index number.

fruits = ["apple", "banana", "cherry"]
print(fruits[0]) # apple
print(fruits[1]) # banana
print(fruits[2]) # cherry

• You can access items in reverser order by referring to the negative index numbers.

fruits = ["apple", "banana", "cherry"]
print(fruits[-1]) # cherry
print(fruits[-2]) # banana
print(fruits[-3]) # apple

• You can change the value of a specific item by referring to its index number.

fruits = ["apple", "banana", "cherry"]
fruits[0] = "kiwi"
print(fruits) # ['kiwi', 'banana', 'cherry']

• You can loop through a list using a for loop.

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)

• You can check if an item exists in a list by using the in keyword.

fruits = ["apple", "banana", "cherry"]
if "apple" in fruits:
  print("Yes, 'apple' is in the fruits list")

• You can get the length of a list by using the len() method.

fruits = ["apple", "banana", "cherry"]
print(len(fruits)) # 3

• You can insert an item into a list by specifying where to insert it.

fruits = ["apple", "banana", "cherry"]
fruits.insert(1, "orange")
print(fruits) # ['apple', 'orange', 'banana', 'cherry']

• You can remove an item from a list by referring to its value.

fruits = ["apple", "banana", "cherry"]
fruits.remove("banana")
print(fruits) # ['apple', 'cherry']

• You can remove an item from a list by referring to its index number.

fruits = ["apple", "banana", "cherry"]
del fruits[0]
print(fruits) # ['banana', 'cherry']

• You can seperate an item from a list by referring to its index number.

fruits = ["apple", "banana", "cherry"]
seperated_fruit = fruits.pop(1)
print(fruits) # ['apple', 'cherry']
print(seperated_fruit) # banana

• You can empty a list by using the clear() method.

fruits = ["apple", "banana", "cherry"]
fruits.clear()
print(fruits) # []

• You can copy a list by using the copy() method.

fruits = ["apple", "banana", "cherry"]
x = fruits.copy()
print(x) # ['apple', 'banana', 'cherry']

• The reverse() method reverses the sorting order of the elements.

fruits = ["apple", "banana", "cherry"]
fruits.reverse()
print(fruits) # ['cherry', 'banana', 'apple']

• You can access tuple items by referring to the index number, inside square brackets.

fruits = ("apple", "banana", "cherry")
print(fruits[1]) # banana

• Negative indexing means start from the end.

fruits = ("apple", "banana", "cherry")
print(fruits[-1]) # cherry

• To determine how many items a tuple has, use the len() method.

fruits = ("apple", "banana", "cherry")
print(len(fruits)) # 3

• You cannot remove items in a tuple.

• You cannot access items in a set by referring to an index, since sets are unordered the items has no index.

• The clear() method empties the set.

fruits = {"apple", "banana", "cherry"}
fruits.clear()
print(fruits) # set()

• To remove an item in a set, use the remove(), or the discard() method.

fruits = {"apple", "banana", "cherry"}
fruits.remove("banana")
print(fruits) # {'cherry', 'apple'}

• The pop() method removes the last item.

fruits = {"apple", "banana", "cherry"}
fruits.pop()
print(fruits) # {'apple', 'banana'}

• Operators are used to perform operations on variables and values.
• Python divides the operators in the following groups:
  • Arithmetic operators
  • Assignment operators
  • Comparison operators
  • Logical operators
  • Identity operators
  • Membership operators
  • Bitwise operators

• Assignment operators are used to assign values to variables.

x = 5
x += 3 # x = x + 3
x -= 3 # x = x - 3
x *= 3 # x = x * 3
x /= 3 # x = x / 3
x %= 3 # x = x % 3
x //= 3 # x = x // 3
x **= 3 # x = x ** 3

• Comparison operators are used to compare two values.

x = 5
y = 3
print(x == y) # False
print(x != y) # True
print(x > y) # True
print(x < y) # False
print(x >= y) # True
print(x <= y) # False

• Logical operators are used to combine conditional statements.

x = 5
print(x > 3 and x < 10) # True
print(x > 3 or x < 4) # True
print(not(x > 3 and x < 10)) # False

• Identity operators are used to compare the objects, not if they are equal, but if they are actually the same object, with the same memory location.

x = ["apple", "banana"]
y = ["apple", "banana"]
z = x
print(x is z) # True
print(x is y) # False
print(x == y) # True
print(x is not z) # False
print(x is not y) # True

• Membership operators are used to test if a sequence is presented in an object.

x = ["apple", "banana"]
print("banana" in x) # True
print("pineapple" not in x) # True

• Bitwise operators are used to compare (binary) numbers.

x = 5
y = 3
print(x & y) # 1
print(x | y) # 7
print(x ^ y) # 6
print(x >> 2) # 1
print(x << 2) # 20

if condition:
    # do something
else:
    # do something else

if condition:
    # do something
elif condition:
    # do something else
else:
    # do something else

for i in range(10):
    print(i)

i = 0
while i < 10:
    print(i)
    i += 1

for i in range(10):
    for j in range(10):
        print(i, j)

• break: used to exit a loop prematurely when a certain condition is met.

for i in range(10):
    if i == 5:
        break
    print(i)

• continue: used to skip the current iteration of a loop when a certain condition is met, but the loop will continue iterating until all the elements in the sequence have been iterated.

for i in range(10):
    if i == 5:
        continue
    print(i)

• else: used to execute a block of code when the condition of a loop is false.

for i in range(10):
    print(i)
else:
    print("Done")

• pass: used to do nothing when a certain condition is met.

for i in range(10):
    if i == 5:
        pass
    print(i)

• A function is a block of code which only runs when it is called.
• You can pass data, known as parameters, into a function.
• A function can return data as a result.

• In Python a function is defined using the def keyword:

def my_function():
    print("Hello from a function")

• To call a function, use the function name followed by parenthesis:

def my_function():
    print("Hello from a function")

my_function()

• Information can be passed into functions as arguments.
• Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want, just separate them with a comma.

def my_function(fname):
    print(fname + " Refsnes")

my_function("Emil")

• A lambda function is a small anonymous function.
• A lambda function can take any number of arguments, but can only have one expression.

x = lambda a : a + 10
print(x(5))

• To let a function return a value, use the return statement:

def my_function(x):
    return 5 * x

print(my_function(3))

• The key function for working with files in Python is the open() function.
• The open() function takes two parameters; filename, and mode.
• There are four different methods (modes) for opening a file:
  • "r" - Read - Default value. Opens a file for reading, error if the file does not exist
  • "a" - Append - Opens a file for appending, creates the file if it does not exist
  • "w" - Write - Opens a file for writing, creates the file if it does not exist
  • "x" - Create - Creates the specified file, returns an error if the file exists

• By default the read() method returns the whole text, but you can also specify how many characters you want to return:

f = open("demofile.txt", "r")
print(f.read(5))

• You can return one line by using the readline() method:

f = open("demofile.txt", "r")
print(f.readline())

• By calling readline() two times, you can read the two first lines:

f = open("demofile.txt", "r")
print(f.readline())
print(f.readline())

• By looping through the lines of the file, you can read the whole file, line by line:

f = open("demofile.txt", "r")
for x in f:
  print(x)

• It is a good practice to always close the file when you are done with it.

f = open("demofile.txt", "r")
print(f.readline())
f.close()

• To write to an existing file, you must add a parameter to the open() function:
  • "a" - Append - will append to the end of the file
  • "w" - Write - will overwrite any existing content

f = open("demofile.txt", "a")
f.write("Now the file has more content!")
f.close()

#open and read the file after the appending:
f = open("demofile.txt", "r")
print(f.read())

• To create a new file in Python, use the open() method, with one of the following parameters:
  • "x" - Create - will create a file, returns an error if the file exist
  • "a" - Append - will create a file if the specified file does not exist
  • "w" - Write - will create a file if the specified file does not exist

f = open("myfile.txt", "x")

• To delete a file, you must import the OS module, and run its os.remove() function:

import os
os.remove("demofile.txt")

• To avoid getting an error, you might want to check if the file exists before you try to delete it:

import os
if os.path.exists("demofile.txt"):
  os.remove("demofile.txt")
else:
  print("The file does not exist")

• To delete an entire folder, use the os.rmdir() method:

import os
os.rmdir("myfolder")

• It is good practice to use the with keyword when dealing with file objects. The advantage is that the file is properly closed after its suite finishes, even if an exception is raised at some point.

with open('workfile') as f:
    read_data = f.read()

try:
    print(x)
except:
    print("An exception occurred")

try:
    print(x)
except NameError:
    print("Variable x is not defined")
except:
    print("Something else went wrong")

try:
    print("Hello")
except:
    print("Something went wrong")
else:
    print("Nothing went wrong")

try:
    print(x)
except:
    print("Something went wrong")
finally:
    print("The 'try except' is finished")