Olivier Brunet

Data geek :)

17 minute read

You’ll find below a summary of the most important key features of the Python programming language. Its clear syntax is illustrated trough simple examples.

I’ve made an other cheat sheet with a focus on the Object Oriented part of Python and on functionnal programming. You can find all of this content in a dedicated anki deck to help you memorizing it.

Cards are composed of a simple challenge, then answer shows the code and its’ result in a dedicated jupyter notebook also available on github.

Design

what are the characteristics of the Python programming language ?

  • interpreted
  • high-level
  • philosophy emphasizes on code readability (indentation)
  • object-oriented
  • dynamically-typed
  • garbage-collected
  • structured (procedural)
  • functional programming
  • described as a “batteries included”

Definitions

Explain what means “interpreted” ?
An interpreter is a computer program that directly executes instructions written in a programming or scripting language, without requiring them previously to have been compiled into a machine language program.

Explain what means “high-level” ?
A high-level programming language is a programming language with strong abstraction from the details of the computer.

Explain what means “object-oriented” ?
Object Oriented programming (OOP) is a programming paradigm that relies on the concept of classes and objects. It is used to structure a software program into simple, reusable pieces of code blueprints (usually called classes), which are used to create individual instances of objects.

Explain what means “dynamically-typed” ?
This means that the Python interpreter does type checking only as code runs, and the type of a variable is allowed to change over its lifetime.

Explain what means “garbage-collected” ?
Garbage collection (GC) is a form of automatic memory management. The garbage collector attempts to reclaim memory which was allocated by the program, but is no longer referenced—also called garbage.

Explain what means “structured (procedural)” ?
Structured programming is a programming paradigm aimed at improving the clarity, quality, and development time by making extensive use of the structured control flows and repetition, block structures. Procedural programming is an otherparadigm based on the concept of the routine or subroutine(series of computational steps).

Explain what means “functional programming” ?
Functional programming is a paradigm where programs are constructed by applying and composing functions (rather than a sequence of imperative statements which update the running state of the program).

Explain why Python is described as “batteries included” ?
This is due to its comprehensive standard library

Basic Types

Boolean
What are the values of a boolean variable ?

True
False

False

Integers
Integer values - all kinds, also in binary or hex

0
-192
0b010
0xF3

243

Strings
a basic string

st = "One\tTwo\nThree"
st

'One\tTwo\nThree'

print(st)

One	Two
Three

a multiple line string

st = """multiple 
lines
long
string"""

st

'multiple \nlines\nlong\nstring'

print(st)

multiple 
lines
long
string

a string with a single quote inside

st = "I'm"
print(st)

I'm

escape a single quote in a string

st = 'I\'m'
print(st)

I'm

Advanced Types / Containers

Lists
an empty list

[]

[]

a list with various values

['x', 11, 8.9]

['x', 11, 8.9]

a list with a single element

['st']

['st']

Tuples
an empty tuple

()

()

a tuple with various elements

11, 'y', 7.4

(11, 'y', 7.4)

a tuple with a single element

('st')

'st'

Dictionnary
an empty dictionnary

{}

{}

a dictionnary with 2 keys

{1: 'one', 'two': 2}

{1: 'one', 'two': 2}

Set
a set with various elements

{1, 1, 2, 3}

{1, 2, 3}

Type conversion
convert a float to an integer

int(1.2)

1

convert an integer to a float

float(1)

1.0

convert an integer to a boolean of value True

bool(1)

True

bool(2)

True

convert an integer to a boolean of value False

bool(0)

False

convert an integer to a string

str(123)

'123'

convert a boolean to a string

str(True)

'True'

convert a set to a list

list({1 ,2, 3})

[1, 2, 3]

convert a string to a list

list("adfs")

['a', 'd', 'f', 's']

convert a list to a string

my_list = ['I', "'", 'l', 'u', 'v']
my_list

['I', "'", 'l', 'u', 'v']

''.join(my_list)

"I'luv"

Iterables objects - properties

/ List Tuple Set Dict
symbol [,] (,) {,} {k: v,}
type ordered ordered unordered unordered
access index index value key
property mutable immutable unique values keys/values

Assignements

multiple assignement of the same value to 3 different variables

a = b = c = 0
a +=1
a, b

(1, 0)

assign 2 different values to 2 differents variables in one line

a, b = 1, 2
a, b

(1, 2)

swap 2 variables’ values

a, b = 2, 8
a, b = b, a
a, b

(8, 2)

list unpacking

x, *y, z = (10, 'a', 'b', 20)
x, z

(10, 20)

string unpacking

i, j = 'ab'
i, j

('a', 'b')

get the last line of a log

*content, last_line = open('/etc/fstab')
last_line

'# UNCONFIGURED FSTAB FOR BASE SYSTEM\n'

decrement a variable

a = 0
a -= 1
a

-1

find the remainder of a division

a = 16
a %= 5
a

1

find the result of a division

a = 16
a /= 5
a

3.2

calculate the quotient of a division

a = 16
a //= 5
a

3

delete a variable

a = 0
del a
a

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

<ipython-input-46-c1e82bdf26d0> in <module>()
      1 a = 0
      2 del a
----> 3 a


NameError: name 'a' is not defined

delete a user-defined objects, lists etc

a, l = 1, [1, 2, 3]
del a, l
a, l

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

<ipython-input-10-4d62949f80bf> in <module>
      1 a, l = 1, [1, 2, 3]
      2 del a, l
----> 3 a, l


NameError: name 'a' is not defined

delete items within lists, dictionaries

l = [1, 2, 3]
del l[1]
l

[1, 3]

last value used by the interpreter

10

10

_

10

ignore a specific value

a, _, b = (1, 2, 3)
_

2

ignore specific values

a, *_, b = (1, 2, 3, 4, 5)
_

[2, 3, 4]

Calculation / Operators

”+ - * / ** % // ~”

complementary

~1

-2

check if two objects are the same object:

a = 'test'
b = a
a is b

True

test if two objects that are equal, but not the same object:

x = ["apple", "banana"]
y = ["apple", "banana"]
x is y

False

x is not y

True

x = y
x is y

True

test if two objects are not the same object:

a = 'test'
b = 'TEST'
a is not b

True

test if an element is in a list

'a' in ['c', 'b', 'a']

True

test if an element is not in a list

'z' not in ['c', 'b', 'a']

True

the opposite of True

not True

False

Or condition with 2 booleans

True or False

True

And condition with 2 booleans

True and True

True

Misc Operators

Source w3schools

Python Arithmetic Operators

png

Python Assignment Operators

png

Python Comparison Operators

png

Python Logical Operators

png

Python Identity Operators

png

Python Membership Operators

png

Python Bitwise Operators

png

0b0011 & 0b1100

0

or bitwise operator

0b0011 | 0b1100

15

Shift left by pushing zeros in from the right and let the leftmost bits fall off

0b0001 << 1

2

Inverts all the bits

~ 0b0001

-2

Sets each bit to 1 if only one of two bits is 1

0b0001 ^ 0b0000

1

test if 2 variables are equal

a, b = 10, 10.0
a == b

True

test if 2 variables are different

a, b = 1, 2
a != b

True

0 == False

True

# everything not equals to 0 is True
0 != True

True

Python Operator Precedence From Python documentation on operator precedence (Section 5.15)

Highest precedence at top, lowest at bottom. Operators in the same box evaluate left to right.

png

concatenate 2 lists

['a', 'b'] + [1, 2]

['a', 'b', 1, 2]

concatenate strings

'this' + 'is' 'hot'

'thisishot'

repeat the same string three times

'test_' * 3

'test_test_test_'

repeat the same list three times

[1, 2, 3] * 3

[1, 2, 3, 1, 2, 3, 1, 2, 3]

loop on the indexes & values of a list

lst = ['a', 'b', 'c']
for i, j in enumerate(lst):
  print(f'index {i} & value {j}')

index 0 & value a
index 1 & value b
index 2 & value c

list integers from 0 to 10

range(10)

range(0, 10)

list(range(10))

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

list integers from 2 to 5

list(range(2, 6))

[2, 3, 4, 5]

list integers with a step of 2

list(range(2, 7, 2))

[2, 4, 6]

Containers Ops

get the min value of a list

lst = [1, 2, 3]
min(lst)

1

get the max value of a list

lst = [1, 2, 3]
max(lst)

3

get the sum of all values of a list

lst = [1, 2, 3]
sum(lst)

6

get the length of a list

lst = [1, 2, 3, 4]
len(lst)

4

test if an element is in a list

lst = [1, 2, 3]
1 in lst

True

lst = [1, 2, 3]
4 in lst

False

get a sorted list

lst = [3, 1, 2, 3]
sorted(lst)

[1, 2, 3, 3]

get a list of tuples from 2 list of the same lenght

lst_1 = [1, 2, 3]
lst_2 = ['a', 'b', 'c']
list(zip(lst_1, lst_2))

[(1, 'a'), (2, 'b'), (3, 'c')]

check if all the value of a list are True

lst = [True, True, True]
all(lst)

True

lst = [False, True, True]
all(lst)

False

check if at least one value of a list is True

lst = [False, False, True]
any(lst)

True

lst = [False, False, False]
any(lst)

False

reverse a list

lst = [3, 1, 2, 3]
list(reversed(lst))

[3, 2, 1, 3]

lst[::-1]

[3, 2, 1, 3]

repeat the same string three times

s = "string__"
s * 3

'string__string__string__'

concatenate two strings

"string___" + " " "test"

'string___ test'

get the index of an element

lst = [3, 1, 2, 3]
lst.index(1)

1

lst = [3, 1, 2, 3]
lst.index(3)

0

count the number of an element

lst = [3, 1, 2, 3]
lst.count(1)

1

lst = [3, 1, 2, 3]
lst.count(3)

2

List Operators

add an element at the end of a list

lst = [3, 1, 2, 3]
lst.append(0)
lst

[3, 1, 2, 3, 0]

remove the 2nd element of a list

lst = [3, 1, 2, 3]
lst.remove(1)
lst

[3, 2, 3]

reverse the elements of a list in the same variable

lst = [4, 1, 2, 3]
lst.reverse()
lst

[3, 2, 1, 4]

sort the elements of a list in the same variable

lst = [4, 1, 2, 3]
lst.sort()
lst

[1, 2, 3, 4]

lst = [4, 1, 2, 3]
sorted(lst)

[1, 2, 3, 4]

lst

[4, 1, 2, 3]

add a list at the end of an other list

lst = [4, 1, 2, 3]
lst.extend([0, 0])
lst

[4, 1, 2, 3, 0, 0]

add an element at the beginning of a list

lst = [4, 1, 2, 3]
lst.insert(0, 7)
lst

[7, 4, 1, 2, 3]

add an element at the 2nd place of a list

lst = [4, 1, 2, 3]
lst.insert(1, 7)
lst

[4, 7, 1, 2, 3]

add an element at the end of a list (with insert or plus)

lst = [4, 1, 2, 3]
lst.insert(-1, 7)
lst

[4, 1, 2, 7, 3]

[4, 1, 2, 3] + [7]

[4, 1, 2, 3, 7]

retrieve the 2nd element of a list and delete it

lst = [4, 7, 1, 2, 3]
lst.pop(1)

7

lst

[4, 1, 2, 3]

Dict Operators

get the value of a corresponding dict key

d = {'a': 1, 'b': 2, 'c': 3}
d['a']

1

list all keys of a dict

d = {'a': 1, 'b': 2, 'c': 3}
d.keys()

dict_keys(['a', 'b', 'c'])

list all values of a dict

d = {'a': 1, 'b': 2, 'c': 3}
d.values()

dict_values([1, 2, 3])

list tuples (k, v) of a dict

d = {'a': 1, 'b': 2, 'c': 3}
d.items()

dict_items([('a', 1), ('b', 2), ('c', 3)])

get the value of a corresponding dict key (with a default one)

d = {'a': 1, 'b': 2, 'c': 3}
d.get('a', 7)

1

d = {'a': 1, 'b': 2, 'c': 3}
d.get('z', 7)

7

get the key of the min value in a dict

d = {'a': 1, 'b': 2, 'c': 0}
min(d, key=d.get)

'c'

removes the item that was last inserted into the dict & returns it

d = {'a': 1, 'b': 2, 'c': 3}
d.popitem()

('c', 3)

d

{'a': 1, 'b': 2}

removes & returns an item in the dict

d = {'a': 1, 'b': 2, 'c': 3}
d.pop('b')

2

d

{'a': 1, 'c': 3}

String Operators

convert a string in uppercase

st = "abcdefgh"
st.upper()

'ABCDEFGH'

st

'abcdefgh'

convert a string in lowercase

st = "ABCDEFGH"
st.lower()

'abcdefgh'

put a capital at the beginning of a string

st = "this is a test"
st.capitalize()

'This is a test'

replace a char by an other in a string

st = "abcdefgh"
st.replace('e', 'Z')

'abcdZfgh'

st = "EabcdEfghE"
st.replace('E', 'Z')

'ZabcdZfghZ'

remove the spaces at the beginning / the end of a string

st = "  this is a test  "
st.strip(' ')

'this is a test'

create a title from a string

st = "this is a test"
st.title()

'This Is A Test'

split lines in a string

st = "this is a 1st line\nthis is a 2nd line"
print(st)
print(st.splitlines())

this is a 1st line
this is a 2nd line
['this is a 1st line', 'this is a 2nd line']

use an f_string with a float

v = 3.14159
f'value of Pi: {v:.2f}'

'value of Pi: 3.14'

test if a string contains only letters

name = "Monica"
name.isalpha()

True

# contains whitespace
name = "Monica Geller"
name.isalpha()

False

# contains number
name = "Mo3nicaGell22er"
name.isalpha()

False

test if a string contains only letters & numbers

name = "Mo3nicaGell22er"
name.isalnum()

True

name = "Mo3nica Gell22er"
name.isalnum()

False

Loops

a simple for loop

for i in range(0, 3):
  print(i**3)

0
1
8

a simple while loop

i = 0
while i < 4:
  print(i**3)
  i += 1

0
1
8
27

a while loop with conditions to break / continue

i = 0
while i < nb:
  if cond1:
    break
  elif cond2:
    continue
  else:
    pass
  i += 1

get user input

st = input("Enter your name: ")
print(f"You're name is {st}", "test", sep="---", end="___")

Enter your name: Me
You're name is Me---test___

Comprehensions

a list comprehension to get even number with only a if (no elese)

[i for i in range(10) if i%2 == 0]

[0, 2, 4, 6, 8]

a list comprehension to list even/odd number (with if and else cases)

["Even" if i%2==0 else "Odd" for i in range(10)]

['Even', 'Odd', 'Even', 'Odd', 'Even', 'Odd', 'Even', 'Odd', 'Even', 'Odd']

a dictionnary comprehension

d = {1: 10, 2: 20, 3: 30}
{k**2: v+1 for (k, v) in d.items()}

{1: 11, 4: 21, 9: 31}

reverse keys/values with a comprehension

d = {1: 10, 2: 20, 3: 30}
{v: k for (k, v) in d.items()}

{10: 1, 20: 2, 30: 3}

with a comprehension get a set of squared element from a list

lst = [1, 2, 3, 3, 2, 1]
{i**2 for i in lst}

{1, 4, 9}

Case, switch statement

a switch statement with default value

def zero():
    return "ZERO"
 
def one():
    return "ONE"
 
def two():
    return "TWO"
 
switcher = {
        0: zero,
        1: one,
        2: two
    }
 
 
def numbers_to_strings(argument):
    """Returns the func from switcher dic"""
    func = switcher.get(argument, "nothing")
    return func()
 
numbers_to_strings(1)

'ONE'

#changing the switch case
switcher[1]=two

numbers_to_strings(1)

'TWO'

switcher[0]()

'ZERO'

Exceptions

Python Built-in Exceptions

print(dir(locals()['__builtins__']))

['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'ZeroDivisionError', '__IPYTHON__', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'all', 'any', 'ascii', 'bin', 'bool', 'breakpoint', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'display', 'divmod', 'dreload', 'enumerate', 'eval', 'exec', 'execfile', 'filter', 'float', 'format', 'frozenset', 'get_ipython', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'range', 'repr', 'reversed', 'round', 'runfile', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']

how to handle exceptions

try:
   # do something
except ValueError:
   # handle ValueError exception
except (TypeError, ZeroDivisionError):
   # handle multiple exceptions
   # TypeError and ZeroDivisionError
except:
   # handle all other exceptions
else:
  # if no exceptoin
finally:
  # for all cases

Recursive functions

factorial recursive function

def fact(n):
  """Calculates the factorial of n"""
  if n == 1:
    return 1
  else:
    return n * fact(n-1)


fact(4)

24

cumulative sum with a recursive function

def cum_sum(n):
  """Calculates the cumulative"""
  if n == 1:
    return 1
  nb = n
  temp = cum_sum(n-1)
  return nb + temp


cum_sum(4)

10

Lambda / map / filter

a simple lambda function with a tuple as input

(lambda x, y: x + y)(2, 3)

5

a lambda function that return the square of a value

(lambda x: x**3)(2)

8

high_ord_func = lambda x, func: x + func(x)
high_ord_func

<function __main__.<lambda>>

high_ord_func(5, lambda x: x*x)

30

def sq(x):
  return x**2

print(map(sq, [1, 2, 3]))

<map object at 0x7ff8e5483550>

list(map(sq, [1, 2, 3]))

[1, 4, 9]

nb = (1, 2, 3, 4)
set((lambda x: x*x, nb))

{(1, 2, 3, 4), <function __main__.<lambda>>}

nb = (1, 2, 3, 4)
res = map(lambda x: x*x, nb)
set(res)

{1, 4, 9, 16}

a = list(range(10))
list(filter(lambda x: x%2 == 0, a))

[0, 2, 4, 6, 8]

PEP 8

Source Real Python

png

_single_leading_underscore
This convention is used for declaring private variables, functions, methods and classes in a module. Anything with this convention are ignored in from module import *.

single_trailing_underscore_
This convention could be used for avoiding conflict with Python keywords or built-ins. You might not use it often.

___double_leading_and_trailing_underscore___
This convention is used for special variables or methods (so-called “magic method”) such as__init__, ___len__. These methods provides special syntactic features or does special things. For example, __file___ indicates the location of Python file, ___eq___ is executed when a == b expression is excuted. A user of course can make custom special method, it is very rare case, but often might modify the some built-in special methods. (e.g. You should initialize the class with ___init___ that will be executed at first when a instance of class is created.)

Files Operation

open a file & print line one by one

with open("file_path/dir", "w/r/a", encoding='utf8') as f:
  for line in f.readlines():
    print(line.split("car"), end=' ')

open a file & read lines

with open("file_path/dir", "w/r/a", encoding='utf8') as f:
  data = f.read().splitlines()

open a file & write line one by one or all together

with open("file_path/dir", "w/r/a", encoding='utf8') as f:
  ...
  f.write() # or
  f.writelines()

Various Tricks

reverse a string

"This is a string"[::-1]

'gnirts a si sihT'

conditional assignement in one line (without ‘else’)

x, y = 0, 10
if y == 10: x = 5
x 

5

concatenate 2 dictionnaries

i = {'a': 1, 'b': 2, 'c': 3}
j = {'d': 4, 'e': 5}
{**i, **j}

{'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

print the path of the ‘os’ module

import os
print(os)

<module 'os' from '/usr/lib/python3.7/os.py'>

conditional assignement in one line with ‘else’ case

y = 10
x = 'a' if y == 10 else 'b'
x

'a'

y = 10
x = 'a' if y != 10 else 'b'
x

'b'

compute the frequency of a list elements

lst = ['a', 'b', 'b', 'c', 'c', 'c']
freq_dict = {}

for i in lst:
  if i not in freq_dict.keys():
    freq_dict[i] = 1
  freq_dict[i] += 1


freq_dict

{'a': 2, 'b': 3, 'c': 4}

get the help of a module function

from math import cosh
help(cosh)

Help on built-in function cosh in module math:

cosh(x, /)
    Return the hyperbolic cosine of x.

retrieve most frequent element of a list

lst = ['a', 'b', 'b', 'c', 'c', 'c']
most_freq = max(set(lst), key=lst.count)
most_freq

'c'

def most_frequent(List):
    return max(set(List), key = List.count)
  
most_frequent(['a', 'b', 'b', 'c', 'c', 'c'])

'c'

declare a Car & a Plane classes, create an instance of one of the two classes depending on an other value

class Car(object):
  def __init__(self, value):
    self.property = "car"
    self.value = value

class Plane(object):
  def __init__(self, param):
    self.property = "plane"
    self.param = param

y == 10
x = (Car if y == 10 else Plane)(22)
isinstance(x, Car)

True

z = (Car if y!= 10 else Plane)(33)
z.property, z.param

('plane', 33)

You may also enjoy

H&M Personalized Fashion 1/2 - EDA

11 minute read

Exploratory analysis of articles, customers, and transactions datasets with recommendation engine specific caracteristics, such as a long tail.

What is federated learning?

10 minute read

A sub-field of ML focusing on decentralization, motivated by issues such as data privacy, data minimization, and data access rights.