Notes on Python

This post is a quick summary of the Python language. This is for version 3 and above.

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  • Python standard library docs can be found at: docs.python.org/3/library
  • Python can be written in a script file with the extension .py or in the interactive shell.
  • To run code just run python my-script.py in the terminal.
  • use print function to print to the console.
  • print function can also take args. example print("You owe", total). This results in You owe 100. It adds space automatically
  • To change the separator use sep arg. example print("You owe $", total, sep=''). This results in You owe $100
  • by default print statements end with a newline to change that use end arg. example print("You owe $", total, end=''). This results in You owe $100 without a newline.
  • use input to get user input. example name = input("Enter your name: \n")
  • an f string in python is like a template string in JavaScript. example print(f"Hello {name}")
  • to separate float with thousands separator use :,.2f in the f string. example print(f"Hello {name}, your total is ${total:,.2f}"). This will make it look like a currency.

Data types

  • Python can infer data type from the value assigned. For example, x = 1 will make x an integer.
  • Here is an example of float. x = 10.50
  • To convert float to an int. use the int function. x = int(10.50)
  • Likewise use float function to convert to float. example x = float(10)
  • Strings can be represented with single or double quotes. example x = "Hello World"
  • To check if a string is a substring of another, use the in keyword. for example: if "e" in "Hello": would evaluate to true
  • example of apostrophe in a string x = "Don't". Using single quote with apostrophe will cause an error.
  • Use + to concatenate strings. example x = "Hello" + "World". Use space between as necessary.
  • Use * to repeat a string. example x = "Hello" * 3
  • Use // to perform a whole integer division. example x = 10 // 3 would yield 3. Whereas x = 10 / 3 would yield 3.33
  • Use % for the remainder of a division. example x = 10 % 3 would yield 1


all conditional statements

if condition1 or condition2: # can also use `and`
    print("do something")
    print("do something else")
    print("cant do something")
  • You can also use the not operator to negate a condition. example if not condition:

List and Loops


  • use append method to add to the list
  • to delete an item either use list.remote(item) or del list[index]
  • use in to check if an item is in the list. example if item in list:
  • to loop use for in loop. example for item in list:
  • use built-in sum function to add all elements of a list.
  • use the range function to generate a list. for example range(0, 10, 2) would result in [0, 2, 4, 6, 8], where 10 is the stop value and 2 is the step/increment value.



  • Dictionaries in python are like objects in JavaScript.
  • use del keyword to delete a key/value pair. example del dict[key]
  • Looking up key that doesnt exists results in a KeyError. Use get method on dictionary for safe lookup. example dict.get(key, default_value)
  • get method can be chained to get nested values. example dict.get(key1).get(key2)
  • if a default_value is not specified then None is returned, this is equivalent to null in JavaScript and evaluates to false in a conditional statement.
  • use the items method on a dictionary to extract both key and value. example for key, value in dict.items():

Virtual Environments

  • use virtual envs to ensure packages installed using pip are isolated from other projects. By default pip installs packages globally.
  • to create a new virtual env use python -m venv my_venv
  • to activate the virtual env use source my_venv/bin/activate
  • to deactivate the virtual env just use deactivate
  • for more details see: https://docs.python.org/3/library/venv.html


  • function definition starts with the def keyword
  • example below:
def greet(name):
    print("Hello", name)
  • variable defined inside a function is local to that function
  • variables defined outside a function are global
  • to return a value from a function use the return keyword


  • class definition starts with the class keyword, followed by name and colon. ex: class MyClass:
  • the __init__ method is the constructor. ex: def __init__(self, name, age):
  • self is always the first param of the init method. It refers to the current instance of the class. somewhat equivalent to this in JavaScript.
  • Methods in classes also have self as the first param.
  • To inherit from a parent class, pass parent class name to the class args. for example: class MyClass(ParentClass):
  • re-defining the methods in the child class will override the parent class methods.
  • use the super() function to call parent classes methods. example: super().__init__(name, age)
  • new keyword is not required to instantiate a class object. for ex: my_obj = MyClass() is sufficient.




  • use try, catch and finally blocks to handle exceptions
  • example:
    # do something
    # handle exception
    # do something
  • to raise custom exceptions use the raise keyword. ex: - raise Exception("Invalid value")
  • to capture the exception object use the as keyword. ex: except FileNotFoundError as e:

File I/O

  • use the open function to open a file. example: file = open("file.txt")
  • use the close method to close the file. example: file.close()
  • use can also use the with keyword to open a file. example: with open("file.txt") as file: This will ensure the file is closed automatically.
  • read method can be used to read the contents of the file. example: file.read()
  • readlines method can be used to read the contents of the file as a list. example: file.readlines() This will return a list of lines in the file that can be looped over using the for-in loop.
  • We can also loop over the file object itself. example: for line in file:
  • To open file in write mode specify the mode as w. example: file = open("file.txt", "w"). This will overwrite the contents of the file, so be careful. use param 'a' to append to the file instead.
  • use the write method on the file to write to the file. example: file.write("Hello")

write mode

  • the os python module provides methods to work with the operating system and file system. example: os.remove("file.txt") will delete the file.
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