Python from A-Z in 10 minutes

Introduction

We all know this: just the moment you begin to like your newly learned programming language, your boss comes around and wants you to work on something completely different. Sure, concepts are similar or even identical - but as we all know: each language has their own peculiarities and for getting started, you are mostly just interested in those and the syntax.

This post wants to achieve exactly that: giving you a brief overview of the most important syntax and concepts in Python as compact as possible.

General

• Python is an interpreted language.
• Python uses indentation instead of brackets, so format your code properly!
• Whitespaces matter
• No strong typing

Comments

# This is a comment

There is no intended way for a multiline comment like for example /* ... */ in Java. However, we can use docstrings in that case. Note, that here the indentation also matters!

def addNumbers(num1, num2, num3):
    """
    A function that returns the sum of
    3 numbers
    """
    return num1 + num2 + num3

Variables

General

• Creation
  

  x = 5
  y = "Hello
  k = 5.2
  

  In case you have a long integer value (e.g. 1.000.000), you can also write it like this 1_000_000.

• Casting

  If you want to specify the data type, this can be done using casting:
  

  y = str(3)
  x = float(7)
  z = int(5)
  

• Type
  The type function returns the data type of the variable
  

  print(type(y))
  

• Illegal names

  Variables can not start with a number, contain a letter or empty space:
  

  2myvar = "John"
  my-var = "John"
  my var = "John"
  

• Naming convention

  The right way to name them is to use snake_case:
  

  myvar = "John"
  my_var = "John"
  _my_var = "John"
  myVar = "John"
  MYVAR = "John"
  myvar2 = "John"
  

Data types

Data type	Example
str	x = "Hello World"
int	x = 20
float	x = 20.5
complex	x = 2j (jis the imaginary part)
bool	x = True (true does not work!)
bytes	x = b"Hello"

Strings

• Multiline
  

  a = """Lorem ipsum dolor sit amet, 
  consectetur adipiscing elit,
  sed do eiusmod tempor incididunt
  ut labore et dolore magna aliqua."""
  

• Strings are arrays

  Strings in Python are arrays of bytes representing unicode characters. Square brackets can be used to access elements of the string:
  

  a = "Hello, World!"
  print(a[1])
  

• Looping over Strings

  Since strings are arrays, we can loop over the single characters using a for loop.
  

  for x in "foo":
  print(x)
  

• Getting the length: len(a)

• Check for a word

  To check for a certain phrase or character in a string, we can use in:
  

  txt = "The best things in life are free!"
  print("free" in txt)
  
  # Used in a if-case
  if "free" in txt:
      print("Yes, 'free' is present.")
  

• Slicing
  

  a = "Hello, World!"
  
  # Get the characters from position x to position y (not included)
  print(a[x:y])
  
  # Get the characters from the start to position x (not included)
  print(a[:x])
  
  # Get the characters from position x, and all the way to the end
  print(a[x:])
  

• Splitting
  

  # returns ['Hello', ' World!']
  print(a.split(","))
  

• Upper/Lower case
  

  a.upper()
  a.lower()
  

• Remove whitespace
  

  a = " Hello, World! "
  
  # returns "Hello, World!"
  print(a.strip())
  

• Replace
  

  print(a.replace("H", "J")) # returns "Jalllo, World!"
  

• Concatenation

  c = a + " bla " + c0

  But be careful with numbers!
  

  # You can't do:
  age = 36
  txt = "My name is John, I am " + age
  print(txt)
  
  # Instead:
  age = 36
  print("My name is John, and I am" + format(age))
  

Operators

The standard ones and also

• x ** y → Exponentiation
• x // y → Floor division (cuts after the comma, e.g. 3 // 2 = 1)

Conditional statements and loops

break and continue are supported.

# if
if b > a:
  print("b is greater than a")
else:
  print("Nix.")

# while
i = 1
while i < 6:
  print(i)
  i += 1

# for
fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)

Data collections

There are no classical arrays (Array = List).

Data Type	Example	Particularities
list	x = ["apple","banana","cherry"]	List items are ordered, changeable, and allow duplicate values. List items are indexed, the first item has index [0], the second item has index [1] etc.
dict	x = {"name" : "John", "age" : 36}	Key-Value-Pairs, ordered (from v3.7 on)
set	x = {"apple", "banana", "cherry"} x = frozenset({"apple","banana","cherry"})	A set is a collection which is both unordered and unindexed. Set items are unordered, unchangeable, and do not allow duplicate values. But: Once a set is created, you cannot change its items, but you can add new items. To prevent this, you can use a frozenset
tuple	("apple", "banana", "cherry")	Tuple items are ordered, unchangeable, and allow duplicate values. Tuples are unchangeable. Can only be accessed with foreach.

Lists

• Access items: thislist[5]

• Change item: thislist[1] = "blackcurrant"

• Inserting

  • Append items: thislist.append("orange")
  • Insert at certain position: thislist.insert(1, "orange")

  • Append elements from another list to the current list: thislist.extend(anotherList)

    The extend() method does not have to append lists, you can add any iterable object (tuples, sets, dictionaries etc.).

• Removing
  

  # remove specified item
  thislist.remove("banana")
  
  # remove specified index
  thislist.pop(1) # you can also leave away the index to remove the last element
  # or:
  del thislist[1]
  
  # empty the list
  thislist.clear()
  
  # delete entire list
  del thislist
  

• Loop through a list

  • foreach
    

    for x in thislist:
        print(x)
    

  • for
    

    for i in range(len(thislist)):
        print(thislist[i])
    

• Sorting
  

  thislist.sort()
  

  You can pass reverse = True as an argument.

• count() → Number of Elements in the List

Dicts

thisdict = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

x = thisdict["model"]
# or:
x = thisdict.get("model")

# return a list of all the keys in the dictionary:
x = thisdict.keys()

# add entry
thisdict["color"] = "white"

# remove entry
thisdict.pop("model")

# remove last item
thisdict.popitem()

Functions

def my_function(fname):
    print(fname + " Doe")
    return fname

my_function("John")

Function definitions cannot be empty, but if you for some reason have a definition with no content, put in the pass statement:

def myfunction():
    pass

Object orientation

• Create a class
  

  class MyClass:
      x = 5
  

• Create an object
  

  p1 = MyClass()
  print(p1.x)
  

• __init__()-Function (= Constructor)
  

  class Person:
      def __init__(self, name, age):
          self.name = name
          self.age = age
  
  p1 = Person("John", 36)
  print(p1.name)
  

• Inheritance
  

  # Superclass
  class Person:
      def __init__(self, fname, lname):
          self.firstname = fname
          self.lastname = lname
  
      def printname(self):
          print(self.firstname, self.lastname)
  
  # Subclass
  class Employee(Person):
      pass