Python
Python is a programming language created by Guido van Rossum.
Getting Started
Introduction
- Python (python.org)
- Learn X in Y minutes (learnxinyminutes.com)
- Regex in python
Hello World
>>> print("Hello, World")
Hello, World
The famous “Hello World” program in Python
Variables
x = 4 # x is of type int
x = "Sally" # x is now of type str
print(x)
Python has no command for declaring a variable.
Data Types
str | Text |
int, float, complex | Numeric |
list, tuple, range | Sequence |
dict | Mapping |
set, frozenset | Set |
bool | Boolean |
bytes, bytearray, memoryview | Binary |
See: Data Types
Slicing String
>>> b = "Hello, World!"
>>> print(b[2:5])
llo
See: Strings
Lists
mylist = []
mylist.append(1)
mylist.append(2)
for x in mylist:
print(x) # prints out 1,2
See: Lists
If Else
a = 200
if a > 0:
print("a is greater than 0")
else:
print("a is not greater than 0")
See: Flow control
Loops
for x in range(6):
if x == 3: break
print(x)
else:
print("Finally finished!")
See: Loops
Functions
>>> def my_function():
... print("Hello from a function")
...
>>> my_function()
Hello from a function
See: Functions
File Handling
with open("myfile.txt", "r", encoding='utf8') as file:
for x in file:
print(x)
See: File Handling
Arithmetic
result = 10 + 30 # => 40
result = 40 - 10 # => 30
result = 50 * 5 # => 250
result = 16 / 4 # => 4
result = 25 % 2 # => 1
result = 5 ** 3 # => 125
Plus-Equals
counter = 0
counter += 10 # => 10
counter = 0
counter = counter + 10 # => 10
message = "Part 1."
# => Part 1.Part 2.
message += "Part 2."
Data Types
Strings
s = "Hello World"
s = 'Hello World'
a = """Multiline Strings
Lorem ipsum dolor sit amet,
consectetur adipiscing elit """
See: Strings
Numbers
x = 1 # int
y = 2.8 # float
z = 1j # complex
>>> print(type(x))
<class 'int'>
Booleans
a = True
b = False
bool(0) # => False
bool(1) # => True
Lists
list1 = ["apple", "banana", "cherry"]
list2 = [True, False, False]
list3 = [1, 5, 7, 9, 3]
list4 = list((1, 5, 7, 9, 3))
See: Lists
Tuple
a = (1, 2, 3)
a = tuple((1, 2, 3))
Similar to List but immutable
Set
set1 = {"a", "b", "c"}
set2 = set(("a", "b", "c"))
Set of unique items/objects
Dictionary
>>> empty_dict = {}
>>> a = {"one": 1, "two": 2, "three": 3}
>>> a["one"]
1
>>> a.keys()
dict_keys(['one', 'two', 'three'])
>>> a.values()
dict_values([1, 2, 3])
>>> a.update({"four": 4})
>>> a.keys()
dict_keys(['one', 'two', 'three', 'four'])
>>> a['four']
4
Casting
Integers
x = int(1) # x will be 1
y = int(2.8) # y will be 2
z = int("3") # z will be 3
Floats
x = float(1) # x will be 1.0
y = float(2.8) # y will be 2.8
z = float("3") # z will be 3.0
w = float("4.2") # w will be 4.2
Strings
x = str("s1") # x will be 's1'
y = str(2) # y will be '2'
z = str(3.0) # z will be '3.0'
Strings
Array-like
>>> a = "Hello, World"
>>> print(a[1])
e
>>> print(a[len(a)-1])
d
Get the character at position 1
Looping
>>> for x in "abc":
... print(x)
a
b
c
Loop through the letters in the word “banana”
Slicing string
┌───┬───┬───┬───┬───┬───┬───┐
| m | y | b | a | c | o | n |
└───┴───┴───┴───┴───┴───┴───┘
0 1 2 3 4 5 6 7
-7 -6 -5 -4 -3 -2 -1
>>> s = 'mybacon'
>>> s[2:5]
'bac'
>>> s[0:2]
'my'
>>> s = 'mybacon'
>>> s[:2]
'my'
>>> s[2:]
'bacon'
>>> s[:2] + s[2:]
'mybacon'
>>> s[:]
'mybacon'
>>> s = 'mybacon'
>>> s[-5:-1]
'baco'
>>> s[2:6]
'baco'
With a stride
>>> s = '12345' * 5
>>> s
'1234512345123451234512345'
>>> s[::5]
'11111'
>>> s[4::5]
'55555'
>>> s[::-5]
'55555'
>>> s[::-1]
'5432154321543215432154321'
String Length
>>> a = "Hello, World!"
>>> print(len(a))
13
The len() function returns the length of a string
Multiple copies
>>> s = '===+'
>>> n = 8
>>> s * n
'===+===+===+===+===+===+===+===+'
Check String
>>> s = 'spam'
>>> s in 'I saw spamalot!'
True
>>> s not in 'I saw The Holy Grail!'
True
Concatenates
>>> s = 'spam'
>>> t = 'egg'
>>> s + t
'spamegg'
>>> 'spam' 'egg'
'spamegg'
Formatting
name = "John"
print("Hello, %s!" % name)
name = "John"
age = 23
print("%s is %d years old." % (name, age))
format() Method
txt1 = "My name is {fname}, I'm {age}".format(fname = "John", age = 36)
txt2 = "My name is {0}, I'm {1}".format("John",36)
txt3 = "My name is {}, I'm {}".format("John",36)
Input
>>> name = input("Enter your name: ")
Enter your name: Tom
>>> name
'Tom'
Get input data from console
Join
>>> "#".join(["John", "Peter", "Vicky"])
'John#Peter#Vicky'
Endswith
>>> "Hello, world!".endswith("!")
True
Lists
Defining
>>> li1 = []
>>> li1
[]
>>> li2 = [4, 5, 6]
>>> li2
[4, 5, 6]
>>> li3 = list((1, 2, 3))
>>> li3
[1, 2, 3]
>>> li4 = list(range(1, 11))
>>> li4
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Generate
>>> list(filter(lambda x : x % 2 == 1, range(1, 20)))
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
>>> [x ** 2 for x in range (1, 11) if x % 2 == 1]
[1, 9, 25, 49, 81]
>>> [x for x in [3, 4, 5, 6, 7] if x > 5]
[6, 7]
>>> list(filter(lambda x: x > 5, [3, 4, 5, 6, 7]))
[6, 7]
Append
>>> li = []
>>> li.append(1)
>>> li
[1]
>>> li.append(2)
>>> li
[1, 2]
>>> li.append(4)
>>> li
[1, 2, 4]
>>> li.append(3)
>>> li
[1, 2, 4, 3]
List Slicing
Syntax of list slicing:
a_list[start:end]
a_list[start:end:step]
Slicing
>>> a = ['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[2:5]
['bacon', 'tomato', 'ham']
>>> a[-5:-2]
['egg', 'bacon', 'tomato']
>>> a[1:4]
['egg', 'bacon', 'tomato']
Omitting index
>>> a[:4]
['spam', 'egg', 'bacon', 'tomato']
>>> a[0:4]
['spam', 'egg', 'bacon', 'tomato']
>>> a[2:]
['bacon', 'tomato', 'ham', 'lobster']
>>> a[2:len(a)]
['bacon', 'tomato', 'ham', 'lobster']
>>> a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[:]
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
With a stride
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[0:6:2]
['spam', 'bacon', 'ham']
>>> a[1:6:2]
['egg', 'tomato', 'lobster']
>>> a[6:0:-2]
['lobster', 'tomato', 'egg']
>>> a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>> a[::-1]
['lobster', 'ham', 'tomato', 'bacon', 'egg', 'spam']
Remove
>>> li = ['bread', 'butter', 'milk']
>>> li.pop()
'milk'
>>> li
['bread', 'butter']
>>> del li[0]
>>> li
['butter']
Access
>>> li = ['a', 'b', 'c', 'd']
>>> li[0]
'a'
>>> li[-1]
'd'
>>> li[4]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range
Concatenating
>>> odd = [1, 3, 5]
>>> odd.extend([9, 11, 13])
>>> odd
[1, 3, 5, 9, 11, 13]
>>> odd = [1, 3, 5]
>>> odd + [9, 11, 13]
[1, 3, 5, 9, 11, 13]
Sort & Reverse
>>> li = [3, 1, 3, 2, 5]
>>> li.sort()
>>> li
[1, 2, 3, 3, 5]
>>> li.reverse()
>>> li
[5, 3, 3, 2, 1]
Count
>>> li = [3, 1, 3, 2, 5]
>>> li.count(3)
2
Repeating
>>> li = ["re"] * 3
>>> li
['re', 're', 're']
Flow control
Basic
a = 5
if a > 10:
print("a is totally bigger than 10.")
elif a < 10:
print("a is smaller than 10.")
else:
print("a is indeed 10.")
One line
>>> a = 330
>>> b = 200
>>> r = "a" if a > b else "b"
>>> print(r)
a
else if
value = True
if not value:
print("Value is False")
elif value is None:
print("Value is None")
else:
print("Value is True")
Loops
Basic
primes = [2, 3, 5, 7]
for prime in primes:
print(prime)
With index
animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
print(i, value)
While
x = 0
while x < 4:
print(x)
x += 1 # Shorthand for x = x + 1
Break
x = 0
for index in range(10):
x = index * 10
if index == 5:
break
print(x)
Continue
for index in range(3, 8):
x = index * 10
if index == 5:
continue
print(x)
Range
for i in range(4):
print(i) # Prints: 0 1 2 3
for i in range(4, 8):
print(i) # Prints: 4 5 6 7
for i in range(4, 10, 2):
print(i) # Prints: 4 6 8
With zip()
name = ['Pete', 'John', 'Elizabeth']
age = [6, 23, 44]
for n, a in zip(name, age):
print('%s is %d years old' %(n, a))
List Comprehension
result = [x**2 for x in range(10) if x % 2 == 0]
print(result)
# [0, 4, 16, 36, 64]
Functions
Basic
def hello_world():
print('Hello, World!')
Return
def add(x, y):
print("x is %s, y is %s" %(x, y))
return x + y
add(5, 6) # => 11
Positional arguments
def varargs(*args):
return args
varargs(1, 2, 3) # => (1, 2, 3)
Keyword arguments
def keyword_args(**kwargs):
return kwargs
# => {"big": "foot", "loch": "ness"}
keyword_args(big="foot", loch="ness")
Returning multiple
def swap(x, y):
return y, x
x = 1
y = 2
x, y = swap(x, y) # => x = 2, y = 1
Default Value
def add(x, y=10):
return x + y
add(5) # => 15
add(5, 20) # => 25
Anonymous functions
# => True
(lambda x: x > 2)(3)
# => 5
(lambda x, y: x ** 2 + y ** 2)(2, 1)
Modules
Import modules
import math
print(math.sqrt(16)) # => 4.0
From a module
from math import ceil, floor
print(ceil(3.7)) # => 4.0
print(floor(3.7)) # => 3.0
Import all
from math import *
Shorten module
import math as m
# => True
math.sqrt(16) == m.sqrt(16)
Functions and attributes
import math
dir(math)
File Handling
Read file
Line by line
with open("myfile.txt") as file:
for line in file:
print(line)
With line number
input = open('myfile.txt', 'r')
for i,line in enumerate(input, start=1):
print("Number %s: %s" % (i, line))
String
Write a string
contents = {"aa": 12, "bb": 21}
with open("myfile1.txt", "w+") as file:
file.write(str(contents))
Read a string
with open('myfile1.txt', "r+") as file:
contents = file.read()
print(contents)
Object
Write an object
contents = {"aa": 12, "bb": 21}
with open("myfile2.txt", "w+") as file:
file.write(json.dumps(contents))
Read an object
with open('myfile2.txt', "r+") as file:
contents = json.load(file)
print(contents)
Delete a File
import os
os.remove("myfile.txt")
Check and Delete
import os
if os.path.exists("myfile.txt"):
os.remove("myfile.txt")
else:
print("The file does not exist")
Delete Folder
import os
os.rmdir("myfolder")
Classes & Inheritance
Defining
class MyNewClass:
'''This is a docstring.'''
pass
# Class Instantiation
my = MyNewClass()
Constructors
class Animal:
def __init__(self, voice):
self.voice = voice
cat = Animal('Meow')
print(cat.voice) # => Meow
dog = Animal('Woof')
print(dog.voice) # => Woof
Method
class Dog:
# Method of the class
def bark(self):
print("Ham-Ham")
charlie = Dog()
charlie.bark() # => "Ham-Ham"
Class Variables
class my_class:
class_variable = "A class variable!"
x = my_class()
y = my_class()
# => A class variable!
print(x.class_variable)
# => A class variable!
print(y.class_variable)
Super() Function
class ParentClass:
def print_test(self):
print("Parent Method")
class ChildClass(ParentClass):
def print_test(self):
print("Child Method")
# Calls the parent's print_test()
super().print_test()
>>> child_instance = ChildClass()
>>> child_instance.print_test()
Child Method
Parent Method
repr() method
class Employee:
def __init__(self, name):
self.name = name
def __repr__(self):
return self.name
john = Employee('John')
print(john) # => John
User-defined exceptions
class CustomError(Exception):
pass
Polymorphism
class ParentClass:
def print_self(self):
print('A')
class ChildClass(ParentClass):
def print_self(self):
print('B')
obj_A = ParentClass()
obj_B = ChildClass()
obj_A.print_self() # => A
obj_B.print_self() # => B
Overriding
class ParentClass:
def print_self(self):
print("Parent")
class ChildClass(ParentClass):
def print_self(self):
print("Child")
child_instance = ChildClass()
child_instance.print_self() # => Child
Inheritance
class Animal:
def __init__(self, name, legs):
self.name = name
self.legs = legs
class Dog(Animal):
def sound(self):
print("Woof!")
Yoki = Dog("Yoki", 4)
print(Yoki.name) # => YOKI
print(Yoki.legs) # => 4
Yoki.sound() # => Woof!
Miscellaneous
Comments
# This is a single line comments.
""" Multiline strings can be written
using three "s, and are often used
as documentation.
"""
''' Multiline strings can be written
using three 's, and are often used
as documentation.
'''
Generators
def double_numbers(iterable):
for i in iterable:
yield i + i
Generators help you make lazy code.
Generator to list
values = (-x for x in [1,2,3,4,5])
gen_to_list = list(values)
# => [-1, -2, -3, -4, -5]
print(gen_to_list)
Handle exceptions
try:
# Use "raise" to raise an error
raise IndexError("This is an index error")
except IndexError as e:
pass # Pass is just a no-op. Usually you would do recovery here.
except (TypeError, NameError):
pass # Multiple exceptions can be handled together, if required.
else: # Optional clause to the try/except block. Must follow all except blocks
print("All good!") # Runs only if the code in try raises no exceptions
finally: # Execute under all circumstances
print("We can clean up resources here")
FAQ
apt install python-is-python3