Software engineering as a service

Custom-built software that fits exactly your needs. Get in touch today.

Python for programmers (part 1)

Python Jun 3, 2020

Python is a weird language when coming from a C-style language (i.e. almost all the popular languages out there). But I say weird in a good way. No curly braces, no variable definitions, not a lot of parenthesis.

And it's quite readable. It's so readable and easy-to-use actually that I got around writing several smallish programs in Python without properly learning the language. By properly I don't mean any official course/certification or something, rather just reading a complete tutorial. I just Googled my way around until I got to the desired outcome.

But this approach has its limitations. So I decided to finally give Python the time it deserves and read the official language tour. This is a summary, a cheatsheet, for anyone else in a similar situation. I plan to split the entire language tour in 3-4 parts. This part covers (3) An Informal Introduction to Python and (4) More Flow Control Tools.

I repeat that this is targeted for people that already know a modern C-style programming language (e.g. Java, Kotlin, Dart, etc) and want to learn Python. Also, this will not cover every feature of the language. I aim to learn about the existance of the most common and basic features at least.  

Basic data types


Integer numbers (e.g. 1, 2 ,3) have type int.
Numbers with fractional part (e.g. 1.0, 2.0, 3.0) have type float.

>>> 10 / 2  # division always returns a float

>>> 10 // 2  # floor division discards the fractional part

>>> 5 ** 2  # 5 squared

>>> 2 * 2.5 #  mixed type operations will have floating point result


Single quotes ('...') or double quotes ("..."), makes not difference.
Span multiple lines using triple-quotes: """...""" or '''...'''.

>>> 'Py' 'thon' # String literals next to each other are concatenated

>>> 3 * 'h' + 'ello' # Concatenated with the +, and repeated with *

>>> len('Hello') # Built-in function returning the length of a string

Characters in a String can be indexed and sliced.
Remember that Strings are immutable, every time you make a change a new instance is created.

>>> word = 'Python'

>>> word[0] # Character in position 0

>>> word[-1] # Last character

>>> word[0:2] # Characters from position 0 (included) to 2 (excluded)

>>> word[:2] # Character from the beginning to position 2 (excluded)

>>> word[4:] # Characters from position 4 (included) to the end

>>> word[-2:] # Characters from the second-last (included) to the end


Might contain items of different types, but usually the items all have the same.
Can be indexed and sliced similarly to a String (described above).
Since mutable, every operation changes the original object.

>>> a_list = [1, 4, 9, 16, 25]

>>> a_list + [36, 49, 64, 81, 100] # Concat lists
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

>>> a_list.append(216)  # Append to a list
[1, 4, 9, 16, 25, 216]

Flow control


Keyword ‘elif’ is short for else if.
Substitute for the switch or case statements found in other languages.

if x < 0:
elif x == 0:


Iterates over the items of any sequence (a list or a string), in the order that they appear.

words = ['ab', 'cd', 'ef']

# Iterate over words
for w in words: 
    print(w, len(w))

# Iterate from 0 to 4
for i in range(5):

# `Else` in `For` statements
for x in range(5):
    if x == 2:
        print('equals 2')
    # Runs when no break occurs
    print('should not happen')

Since there are not indexes, if you need to modify the collection you are iterating through you need do something like this:

# Iterate over a copy
for user, status in users.copy().items():
    if status == 'inactive':
        del users[user]

# Create a new collection
active_users = {}
for user, status in users.items():
    if status == 'active':
        active_users[user] = status


def hey(n):
    """This is the method documentation."""
    print('Hello world')


Can be used is as a placeholder for a function or conditional body.

def do_something():
    pass   # Remember to implement this!

Default values

def optionaly_greet(should_greet, greeting='Hey there!'):
    if should_greet:

Default values are evaluated at the point of function definition and only once.

def f(a, L=[]):
    return L


# This will print:
# [1]
# [1, 2]
# [1, 2, 3]

Keyword arguments

Call methods with arguments in keyword=value format.

def optionaly_greet(should_greet, greeting='Hey there!'):
    if should_greet:
# All these calls have the same result
optionaly_greet(True, 'Hello!')
optionaly_greet(True, greeting='Hello!')
optionaly_greet(greeting='Hello!', should_greet=True)

Special arguments

A parameter of the form *name receives a tuple containing the positional arguments beyond the formal parameter list. A parameter **name receives a dictionary (explained later) containing all keyword arguments except for those corresponding to a formal parameter.

def special_one(formal_parameter, *arguments_tuple, **keywords_map):
    for arg in arguments_tuple:
    print("-" * 40)
    for kw in keywords_map:
        print(kw, ":", keywords_map[kw])

special_one("Hello", "World", not_fomral_paramter="!!")

# Hello
# World
# ----------------------------------------
# not_formal_parameter : !!

Type of arguments

There is a way to define how the parameters should be passed: Positional-or-Keyword Arguments, Positional-Only Parameters, Keyword-Only Arguments.

Arguments unpacking

Call with the *-operator to unpack the arguments out of a list or tuple.

>>> args = [3, 6]
>>> list(range(*args)) # Same as range(3, 6)
[3, 4, 5]


Syntactically restricted to a single expression.

pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
pairs.sort(key=lambda pair: pair[1])


First line should always be a short, concise summary of the object’s purpose.
Second line should be blank.
Following lines should be one or more paragraphs describing the object’s calling conventions, its side effects, etc.

def my_function():
    """Do nothing, but document it.

        No, really, it doesn't do anything.

Coding style

There is a single popular coding style called PEP 8 (summary).

Continue to part 2...


Great! You've successfully subscribed.
Great! Next, complete checkout for full access.
Welcome back! You've successfully signed in.
Success! Your account is fully activated, you now have access to all content.