Python Cheatsheet

This cheat sheet is based on The Python Mega Course by Ardit Sulce

Course link: http://udemy.com/the-python-mega-course

Python Data Types

Integers are used to represent whole numbers:

rank = 10
eggs = 12
people = 3

Floats represent decimal numbers:

temperature = 10.2
rainfall = 5.98
elevation = 1031.88

Strings represent text:

message = "Welcome to our online shop!"
name = "John"
serial = "R001991981SW"

Lists represent arrays of values that may change during the course of the
program:

members = ["Sim Soony", "Marry Roundknee", "Jack Corridor"]
pixel_values = [252, 251, 251, 253, 250, 248, 247]

Dictionaries represent pairs of keys and values:

phone_numbers = {"John Smith": "+37682929928", "Marry Simpons": "+423998200919"}
volcano_elevations = {"Glacier Peak": 3213.9, "Rainer": 4392.1}

Keys of a dictionary can be extracted with:

phone_numbers.keys()

Values of a dictionary can be extracted with:

phone_numbers.values()

Tuples represent arrays of values that are not to be changed during the course
of the program:

vowels = ('a', 'e', 'i', 'o', 'u')
one_digits = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

You can get a list of attributes of a data type has using:

dir(str)
dir(list)
dir(dict)

You can get a list of Python builtin functions using:

dir(__builtins__)

You can get the documentation of a Python data type using:

help(str)
help(str.replace)
help(dict.values)

Operations with Data Types
Lists, strings, and tuples have a positive index system:

["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
   0      1      2     3       4      5      6

And they have a negative index system as well:

["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
 -7   -6    -5  -4   -3   -2   -1

In a list, the 2nd, 3rd, and 4th items can be accessed with:

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[1:4]
Output: ['Tue', 'Wed', 'Thu']

First three items of a list:

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[:3]
Output:['Mon', 'Tue', 'Wed']

Last three items of a list:

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[-3:]
Output: ['Fri', 'Sat', 'Sun']

Everything but the last:

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[:-1]
Output: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']

Everything but the last two:

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[:-2]
Output: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri']

A dictionary value can be accessed using its corresponding dictionary key:

phone_numbers = {"John":"+37682929928","Marry":"+423998200919"}
phone_numbers["Marry"]
Output: '+423998200919'

Functions and Conditionals

You can define functions with:

def cube_volume(a):
    return a * a * a

Write if-else conditionals:

message = "hello there"
if "hello" in message:
    print("hi")
else:
    print("I don't understand")

Write if-elif-else conditionals:

message = "hello there"
if "hello" in message:
    print("hi")
elif "hi" in message:
    print("hi")
elif "hey" in message:
    print("hi")
else:
    print("I don't understand")
else:
    print("No")

Use the and operator to check if both conditions are True at the same time:

x = 1
y = 1
if x == 1 and y==1:
    print("Yes")

Use the or operator to check if at least one condition is True:

x = 1
y = 2
if x == 1 or y==2:
    print("Yes")
else:
    print("No")

Check if a value is of a particular type with isinstance:

isinstance("abc", str)
isinstance([1, 2, 3], list)

or directly:

type("abc") == str
type([1, 2, 3]) == list

Processing User Input
A Python program can get user input via the input function:
The input function halts the execution of the program and gets text input from
the user:

name = input("Enter your name: ")

The input function converts any input to a string, but you can convert it back to
int or float:

experience_months = input("Enter your experience in months: ")
experience_years = int(experience_months) / 12

You can also format strings with:

name = "Sim"
experience_years = 1.5
print("Hi {}, you have {} years of experience".format(name, experience_years))
Output: Hi Sim, you have 1.5 years of experience

Python Loops
A for-loop is useful to repeatedly execute a block of code.
You can create a for-loop like so:

for letter in 'abc':
    print(letter.upper())

Output: A B C

As you can see, the for-loop repeatedly converted all the items of 'abc' to uppercase. The name after for (e.g. letter ) is just a variable name.

You can loop over dictionary keys as follows:

phone_numbers = {"John Smith":"+37682929928","Marry Simpons":"+423998200919"}
for value in phone_numbers.keys():
    print(value)

Output:

John Smith Marry Simpsons

You can loop over dictionary values:

phone_numbers = {"John Smith":"+37682929928","Marry Simpons":"+423998200919"}
for value in phone_numbers.values():
print(value)

Output:
+37682929928 +423998200919

You can loop over dictionary items:

phone_numbers = {"John Smith":"+37682929928","Marry Simpons":"+423998200919"}
for key, value in phone_numbers.items():
print(key, value)

Output:
John Smith +37682929928
Marry Simpons +423998200919

We also have while-loops. The code under a while-loop will run as long as the
while-loop condition is true:

while datetime.datetime.now() < datetime.datetime(2090, 8, 20, 19, 30, 20):
   print("It's not yet 19:30:20 of 2090.8.20")

The loop above will print out the string inside print() over and over again until
the 20th of August, 2090.

List Comprehensions
A list comprehension is an expression that creates a list by iterating over
another container.

A basic list comprehension:

[i*2 for i in [1, 5, 10]]

Output: [2, 10, 20]

List comprehension with if condition:

[i*2 for i in [1, -2, 10] if i>0]

Output: [2, 20]
List comprehension with an if and else condition:

[i*2 if i>0 else 0 for i in [1, -2, 10]]

Output: [2, 0, 20]

More on Functions
Functions can have more than one parameter:

def volume(a, b, c):
    return a * b * c

Functions can have default parameters (e.g. coefficient ):

def converter(feet, coefficient = 3.2808):
    meters = feet / coefficient
    return meters
print(converter(10))

Output: 3.0480370641306997
Arguments can be passed as non-keyword (positional) arguments (e.g. a )
or keyword arguments (e.g. b=2 and c=10 ):

def volume(a, b, c):
    return a * b * c
print(volume(1, b=2, c=10))

An args parameter allows the function to be called with an arbitrary
number of non-keyword arguments:

def find_max(*args):
   return max(args)
print(find_max(3, 99, 1001, 2, 8))

Output: 1001
A *kwargs parameter allows the function to be called with an arbitrary
number of keyword arguments:
def find_winner(**kwargs):
return max(kwargs, key = kwargs.get)
print(find_winner(Andy = 17, Marry = 19, Sim = 45, Kae = 34))
Output: Sim

File Processing in Python
You can read an existing file with Python:

with open("file.txt") as file:
    content = file.read()

You can create a new file with Python and write some text on it:

with open("file.txt", "w") as file:
    content = file.write("Sample text")

You can append text to an existing file without overwriting it:

with open("file.txt", "a") as file:
    content = file.write("More sample text")

You can both append and read a file with:

with open("file.txt", "a+") as file:
    content = file.write("Even more sample text")
    file.seek(0)
    content = file.read()

Python Modules
Builtin objects are all objects that are written inside the Python interpreter in C language. Builtin modules contain builtins objects. Some builtin objects are not immediately available in the global namespace. They are parts of a builtin module. To use those objects the module needs to be imported first. E.g.:

import time
time.sleep(5)

A list of all builtin modules can be printed out with:

import sys
sys.builtin_module_names

Standard libraries is a jargon that includes both builtin modules written in C and also modules written in Python. Standard libraries written in Python reside in the Python installation directory as .py files. You can find their directory path with sys.prefix. Packages are a collection of .py modules. Third-party libraries are packages or modules written by third-party persons (not the Python core development team). Third-party libraries can be installed from the terminal/command line:

Windows:
pip install pandas

if that does not work, then use:

python -m pip install pandas

Mac and Linux:

pip3 install pandas

if that does not work, then use:

python3 -m pip install pandas

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