Basic Python

In this post, I’ll show you everything you need to know to start coding in Python and score high on basic Python tests I wish I had 4 years ago. Let’s dive right in!

Comments

Inline comment:

# This is a comment

Multiline comment:

# This is a
# multiline comment

Code with a comment:

a = 1  # Initialization

Please note the two spaces in front of the comment.

Math Operators

From highest to lowest precedence:

Operators	Operation	Example
**	Exponent	`2 ** 3 = 8`
%	Modulus/Remainder	`22 % 8 = 6`
//	Integer Division	`22 // 8 = 2`
/	Division	`22 / 8 = 2.75`
*	Multiplication	`3 * 3 = 9`
-	Subtraction	`5 - 2 = 3`
+	Addition	`2 + 2 = 4`

Examples of expressions:

>>> 2 + 3 * 6
# 20

>>> (2 + 3) * 6
# 30

>>> 2 ** 8
#256

>>> 23 // 7
# 3

>>> 23 % 7
# 2

>>> (5 - 1) * ((7 + 1) / (3 - 1))
# 16.0

Data Types

Data Type	Examples
Integers	`-2, -1, 0, 1, 2, 3, 4, 5`
Floating-Point Numbers	`-1.25, -1.0, -0.5, 0.0, 0.5, 1.0, 1.25`
Strings	`'a', 'aa', 'aaa', 'hello!', '11 cats', '123'`

Important: Note that ‘123’ is still a string.

Variables

You can name a variable anything as long as it obeys the following rules:

It can be only one word.

>>> # Bad
>>> my_variable = 'hello'

>>> # Good
>>> var = 'hello'

It can use only letters, numbers, and the underscore (_) character.

>>> # Bad
>>> %$@variable = 'hello'

>>> # Good
>>> my_var = 'hello'

>>> # Good
>>> my_var_2 = 'hello'

It can’t begin with a number.

>>> # This won't work
>>> 23_var = 'hello'

Variable names starting with an underscore (_) are considered as “unuseful”.

>>> # _spam should not be used again in the code
>>> _spam = 'hello'

The `print()` Function

The print() function writes the value of the argument(s) it is given. It handles multiple arguments, floating-point numbers, integers, and strings. Strings are printed without quotes, and a space is inserted when items are separated by a comma:

>>> print('hello world!')
# Output: hello world!

>>> a = 1
>>> print('hello world!', a)
# Output: hello world! 1

Concatenate items

>>> # Good
>>> a = 'joe' 
>>> print('hello world!' + a)
# Output: hello world!joe

>>> # Bad
>>> a = 1  # Integer(int)
>>> print('hello world!' + a)  # String(str) + Integer(int)
# TypeError: can only concatenate str (not "int") to str

The `input()` Function

This function takes the input from the user and converts it into a string:

>>> print('What is your name?')   # Ask for their name
>>> my_name = input()
>>> print('Hi', my_name)
# What is your name?
# Martha
# Hi, Martha

input() can also set a default message without using print():

>>> my_name = input('What is your name? ')  # Default message
>>> print('Hi', my_name)
# What is your name? Martha
# Hi, Martha

The `str()`, `int()`, and `float()` Functions

These functions allow you to change the type of a variable. For example, you can transform an integer or float to a string:

>>> str(29) # str
# '29'

>>> str(-3.14) # str
# '-3.14'

Or from a string to an integer or float:

>>> int('11') # Integer
# 11

>>> float('3.14') # Decimal
# 3.14

Conditions

To do this in Python, you use the if, else, and elif keywords. These two keywords are called conditionals.

Use the if keyword in Python

>>> if condition:
...     Indented block of decision to make if condition is true

>>> # Example
>>> team_brian = 99
>>> team_jack = 59
>>> if team_brian > team_jack:
...     print("Team Brian won the league") # Output: Team Brian won the league

Important: If the condition in the if statement is not met, nothing happens.

>>> team_brian = 59
>>> team_jack = 99
>>> if team_brian > team_jack:
...     print("Team Brian won the league")
        # Output: Nothing will output because if statement is not met

Use the else keyword in Python.

Since nothing happens if the condition in an if statement is not met, you can catch that with an else statement.

>>> if condition:
...     Indented block of decision to make if condition is true
... else:
...     Indented block of decision to make if condition is not true


>>> # Example
>>> team_brian = 59
>>> team_jack = 99
>>> if team_brian > team_jack:
...     print("Team Brian won the league")
... else:
...     print("Team Jack won the league")
        # Output: Team Jack won the league

Use the elif keyword in Python

Another conditional keyword in Python is elif, which you can put in between an if and else. elif is more specific than else.

>>> # Example
>>> team_brian = 59
>>> team_jack = 89
>>> team_lilian = 99
>>> if team_brian > team_jack:
...     print("Team Brian won the league")
... elif team_lilian > team_brian:
...     print("Team Lilian won the league")
... else:
...     print("Team Jack won the league")
        # Output: Team Lilian won the league

Import

Basically, import allows you to use pre-written code of someone else who already wrote the code and is willing to share it.

This code would have to be written if the random module was not used

# This code doesn't work as it is only a small portion of the random module 

def randrange(self, start, stop=none, step=_one):
    """Choose a random item from range(stop) or range(start, stop[, step]).
    Roughly equivalent to `choice(range(start, stop, step))` but
    supports arbitrarily large ranges and is optimized for common cases.
    """

    # This code is a bit messy to make it fast for the
    # Common case while still doing adequate error checking.
    istart = _index(start)
    if stop is none:
        # We don't check for "step != 1" because it hasn't been
        # Type checked and converted to an integer yet.
        if step is not _one:
            raise TypeError("Missing a non-None stop argument")
        if istart > 0:
            return self._randbelow(istart)
        raise ValueError("Empty range for randrange()")

    # Stop argument supplied.
    istop = _index(stop)
    width = istop - istart
    istep = _index(step)
    # Fast path.
    if istep == 1:
        if width > 0:
            return istart + self._randbelow(width)
        raise ValueError(f"Empty range in randrange({start}, {stop})")

        # Non-unit step argument supplied.
        if istep > 0:
            n = (width + istep - 1) // istep
        elif istep < 0:
            n = (width + istep + 1) // istep
        else:
            raise ValueError("Zero step for randrange()")
        if n <= 0:
            raise ValueError(f"Empty range in randrange({start}, {stop}, {step})")
        return istart + istep * self._randbelow(n)


def randint(self, a, b):
        """Return random integer in range [a, b], including both end points.
        """

        return self.randrange(a, b+1)

random = random.randint(1,100) # Generate random number between 1 and 100
print(random)

But if the random module was used, it will be much easier and more efficient to write your code.

import random
random = random.randint(1,100) # Generate random number between 1 and 100
print(random)

While Loop Statements

The while statement is used for repeated execution as long as an expression is True:

>>> spam = 0
>>> while spam < 5:
...     print('Brian')
...     spam = spam + 1
...
# Output:
# Brian
# Brian
# Brian
# Brian
# Brian

Break Statements

If the execution reaches a break statement, it immediately exits the while loop’s clause:

>>> while True: # Infinite loop
...     name = input('Please type your name: ')
...     if name == 'your name':
...         break
...
>>> print('Thank you!')
# Please type your name: your name
# Thank you!

Continue Statements

When the program execution reaches a continue statement, the program execution immediately jumps back to the start of the loop.

>>> while True:
...     name = input('Who are you? ')
...     if name != 'Joe':
...         continue
...     password = input('Password? (It is a fish.): ')
...     if password == 'swordfish':
...         break
...
>>> print('Access granted.')
# Who are you? Charles
# Who are you? Debora
# Who are you? Joe
# Password? (It is a fish.): swordfish
# Access granted.

Cool Things to Know

Concatenation and Replication

String concatenation:

>>> 'Alice' 'Bob'
# 'AliceBob'

String replication:

>>> 'Alice' * 5
# 'AliceAliceAliceAliceAlice'

The `end` Keyword

The keyword argument end can be used to avoid the newline after the output or end the output with a different string:

phrase = ['Printed', 'with', 'a', 'dash', 'in', 'between']
>>> for word in phrase:
...     print(word, end='-')
...
# Printed-with-a-dash-in-between-

The `sep` Keyword

The keyword sep specifies how to separate the objects if there is more than one:

print('Jack', 'Lilian', 'Charlotte', sep=',')
# Jack,Lilian,Charlotte

Conclusion

Now I’d Like to Hear From You:

How did you think of my cheat sheet?
What is your favorite thing about Python?

Comment below to let me know!

Next Step

Visit my blog next week where I will dive deep into Python!

Source

Summarized from the Python 3 tutorial.

Basic Python#

Comments#

Math Operators#

Data Types#

Variables#

The print() Function#

The input() Function#

The str(), int(), and float() Functions#

Conditions#

Import#

While Loop Statements#

Break Statements#

Continue Statements#

Cool Things to Know#

Concatenation and Replication#

The end Keyword#

The sep Keyword#

Conclusion#

Now I’d Like to Hear From You:#

Next Step#

Source#