Python - Essentials

Learn pythong and create a simple app that takes in github username and returns the list of repositories sample: https://qasinow.pythonanywhere.com/

• Run the final app
  • clone repo
  • cd python
  • pip install flask
  • FLASK_APP=script.py flask run

Python

• Python is a high-level, interpreted, interactive and object-oriented scripting language.

Variables

• Variables are containers for storing data values.
• Unlike other programming languages, Python has no command for declaring a variable.

x = 5
y = "John"
print(x)

Data Types

• Some of data types in Python
  • Numbers `int, float, complex
  • String str
  • List list
  • Tuple tuple
  • Set set
  • Dictionary dict
  • Boolean bool
  • None NoneType

Numbers

Numbers in python are are represented by int, float, complex

• int : integer 1, 2, 3, 4, 5
• float : floating point number 1.0, 2.0, 3.0, 4.0, 5.0
• complex : complex number 1+2j, 2+3j, 3+4j, 4+5j, 5+6j
• type(5) -> int
• type(5.0) -> float
• type(5+6j) -> complex

Strings

Strings are arrays of bytes representing unicode characters, str

• str : string "hello", "world", "python", "programming"
• type("hello") -> str
• type('hello') -> str
• type("""hello""") -> str

Lists

Lists are used to store multiple items in a single variable, list

• list : list [1, 2, 3, 4, 5]
• type([1, 2, 3, 4, 5]) -> list
• type([1, 2.0, 3+4j, "hello", [1, 2, 3]]) -> list
• type([]) -> list

some common list methods

list -> [2,'qasim', 3, 4, 5]

• len([2,'qasim', 3, 4, 5]) -> 5
• list.append(6) -> [2,'qasim', 3, 4, 5, 6]
• list.remove('qasim') -> [2, 3, 4, 5, 6]
• list.pop() -> [2, 3, 4, 5]
• list.insert(1, 'qasim') -> [2, 'qasim', 3, 4, 5]
• list.sort() -> [2, 3, 4, 5, 'qasim']
• list.reverse() -> ['qasim', 5, 4, 3, 2]
• list.clear() -> []
• list.copy() -> [2, 'qasim', 3, 4, 5]
• list.count(2) -> 1

Tuples

Tuples are used to store multiple items in a single variable, they are different from lists in the sense that they are immutable i.e they cannot be changed, tuple

• tuple : tuple (1, 2, 3, 4, 5)
• type((1, 2, 3, 4, 5)) -> tuple
• type((1, 2.0, 3+4j, "hello", [1, 2, 3])) -> tuple

Some common tuple methods

tuple -> (2,'qasim', 3, 4, 5)

• len((2,'qasim', 3, 4, 5)) -> 5
• tuple.count(2) -> 1
• tuple.index(2) -> 0
• tuple[0] -> 2
• tuple[1] -> qasim>
• tuple[2] -> 3

Sets

Sets are used to store multiple items in a single variable, they are unordered and unindexed, set, repeated items are not allowed

• set : set {1, 2, 3, 4, 5}
• type({1, 2, 3, 4, 5}) -> set
• type({1, 2.0, 3+4j, "hello", [1, 2, 3]}) -> set

Some common set methods

set -> {2,'qasim', 3, 4, 5}

• len({2,'qasim', 3, 4, 5}) -> 5
• set.add(6) -> {2,'qasim', 3, 4, 5, 6}
• set.remove('qasim') -> {2, 3, 4, 5, 6}
• set.pop() -> {2, 3, 4, 5}
• set.clear() -> set()
• set.copy() -> {2, 'qasim', 3, 4, 5}
• set.discard(2) -> {'qasim', 3, 4, 5}

Dictionaries

Dictionaries are used to store data values in key:value pairs, dict

• dict : dict {"name": "qasim", "age": 25, "city": "Ottawa"}
• type({"name": "qasim", "age": 25, "city": "Ottawa"}) -> dict
• type({"name": "qasim", "age": 25, "city": "Ottawa", "hobbies": ["coding", "reading", "gaming"]}) -> dict

Some common dictionary methods

dict -> {"name": "qasim", "age": 25, "city": "Ottawa"}

• len({"name": "qasim", "age": 25, "city": "Ottawa"}) -> 3
• dict["name"] -> qasim
• dict.get("name") -> qasim
• dict.keys() -> dict_keys(['name', 'age', 'city'])
• dict.values() -> dict_values(['qasim', 25, 'Ottawa'])
• dict.items() -> dict_items([('name', 'qasim'), ('age', 25), ('city', 'Ottawa')])
• dict.pop("name") -> qasim

Booleans

Booleans represent one of two values: True or False

• bool : bool True, False
• type(True) -> bool
• type(False) -> bool

Some common boolean methods

• bool(0) -> False
• bool(1) -> True
• bool(2) -> True
• bool(3) -> True

Comparison Operators

• == : equal
• != : not equal
• > : greater than
• < : less than
• >= : greater than or equal to
• <= : less than or equal to

Logical Operators

• and : and
• or : or
• not : not
• & : and
• | : or
• ~ : not

if, elif, else Statements

if condition:
    # code
elif condition:
    # code
else:
    # code

for Loops

for i in range(5):
    print(i)

for val in "string":
    if val == "i":
        break
    print(val)

while Loops

i = 1
while i < 6:
    print(i)
    i += 1
while True:
    print("hello")
    break

list comprehension

list comprehension is a way to create lists in python

new_list = [i for i in old_list]
new_list = [i.upper() for i in old_list]
new_list = [i for i in old_list if i != 0]

functions

functions are a block of code that only runs when it is called there are many ways to define a function

def my_function(name):
    print(f"hello {name}")
def my_function(name="qasim"):
    print(f"hello {name}")
def my_function(*args):
    while args:
        print(args.pop())
def my_function(**kwargs):
    for key, value in kwargs.items():
        print(f"{key} : {value}")

lambda expressions

lambda expressions are used to create anonymous functions and are used to create small functions

x = lambda a : a + 10
print(x(5))

my_function = lambda a : a + 10
print(my_function(5))
print((lambda a : a + 10)(5))

map and filter

map and filter are used to apply a function to a list of elements map will return a list of the results after applying the given function to each item of a given iterable filter will return a list of the elements of the iterable for which the function returns True

my_list = [1, 2, 3, 4, 5]
new_list = list(map(lambda x: x * 2, my_list))
new_list = list(filter(lambda x: x % 2 == 0, my_list))

methods

• dir() : returns a list of all the methods and properties of the object
• help() : returns the documentation of the object
• type() : returns the type of the object
• id() : returns the unique id of the object

nested statements and scope

• LEGB Rule
  • L : Local
  • E : Enclosing
  • G : Global
  • B : Built-in the scope of a variable is the context in which it is defined.

x = 300
def my_function():
    x = 200
    print(x)
print(x) # 300
def my_function():
    global x
    x = 200
    print(x) # 200

file I/O

to use files in python, you have to open them first, using the built-in open() function

f = open("demofile.txt", "r")
print(f.read())
f.close()

REPL

• Read : read the user input
• Evaluate : evaluate the user input
• Print : print the result
• Loop : loop the above process until the user quits

helpful funvtions

• type()
• dir()
• help()

String functions

• len()
• lower()
• strip()
• split()

List functions

• append()
• remove()
• set()
• sort()
• join() " * ".join(list) -> a * b * c where list = [a, b, c]

list single line for loop

• new_list = [i.upper() for i in old_list]
• set_list = {i.upper() for i in old_list}
• dict_list = {i: i.upper() for i in old_list}

creating copy of list

• new_list = old_list # here the changes in new_list will reflect in old_list
• new_list = old_list[:] # here the changes in new_list will not reflect in old_list
• new_list = old_list.copy() # here the changes in new_list will not reflect in old_list
• new_list = list(old_list) # here the changes in new_list will not reflect in old_list

zip function

• zip([1, 2, 3], [4, 5, 6]) -> [(1, 4), (2, 5), (3, 6)]
• zip combine the elements of two lists in a tuple

Packages

Standard Library

• import random
• import math
• import os
• import sys
• import json full list of standard library: https://docs.python.org/3/library/

Creating and Using module

• cerate folder mkdir my_module
• create file touch my_module/my_module.py
• create file touch my_module/__init__.py
• from my_module import my_module
• my_module.my_function()

os

• os.getcwd() gets current working directory
• os.chdir('path') change directory
• os.listdir() list all files and folders in current directory

sys

• sys.path list of all the directories where python looks for modules

• sys.argv is a list in Python, which contains the command-line arguments passed to the script.

• sys.exit() exit from python

basic command line arguments

• python3 my_script.py arg1 arg2 arg3

import sys

print('Number of arguments:', len(sys.argv), 'arguments.')

Testing in Python

• unit testing : testing individual units or components of a software
• integration testing : testing the combination of two or more units
• functional testing : testing the functionality of the software
• end-to-end testing : testing the complete flow of the software

assert

• assert 1 == 1 # no output
• assert 1 == 2 # AssertionError

unittest

• import unittest
• unittest.TestCase is a class that allows us to create test cases

Sample unittest

import unittest
class TestStringMethods(unittest.TestCase):
    def test_upper(self):
        self.assertEqual('foo'.upper(), 'FOO')

    def test_isupper(self):
        self.assertTrue('FOO'.isupper())
        self.assertFalse('Foo'.isupper())

asserts

• assertEqual(a, b) checks a and b are equal
• assertNotEqual(a, b) checks a and b are not equal
• assertTrue(x) checks x is True
• assertFalse(x) checks x is False
• assertIs(a, b) checks a is b
• assertIsNot(a, b) checks a is not b
• assertIsNone(x) checks x is None
• assertIsNotNone(x) checks x is not None

Web Frameworks

• Django: high-level web framework
• Flask: micro web framework
• Pyramid: open source web framework
• Bottle: simple web framework

Flask

• pip install flask
• making a simple hello world with welcome animated message

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'

• flask run to run the server, it will look for app.py and run the server on port : 5000
• if the file name is different then FLASK_APP=script.py flask run

Routess with parameters

@app.route('/user/<username>')
def show_user_profile(username):
    # show the user profile for that user
    return 'User %s' % username

Templates

• we can pass arguments to the templates
• render_template('index.html', name=name)
• the template file should be in templates folder
• inorder to render the passed argument in the template file we use {{ name }}

@app.route('/hello/<user>')
def hello_name(user):
    return render_template('hello.html', name=user)

<h1> welcome {{ name }} </h1>

Templating syntax

• {{ variable }} : to print the variable
• {% for i in list %} {{ i }} {% endfor %} : for loop
• {% if condition %} {{ variable }} {% endif %} : if condition
• {% block content %} {% endblock %} : block of content
• {% extends "layout.html" %} : extending the layout file
• {% include "header.html" %} : including the header file

Template syntax - 2

• {% set name = "qasim" %} : setting the variable
• {% macro render_title(title) %} <title>{{ title }}</title> {% endmacro %} : creating a macro
• {{ render_title("Home") }} : using the macro
• the diffrence between {{ }} and {% %} is that {{ }} is used to print the variable and {% %} is used to write the logic

the requests module

• requests module is used to make the http requests
• pip install requests

import requests

@app.route('/repos/<username>')
def repos(username):
    r = requests.get(f'https://api.github.com/users/{username}/repos')
    return r.json()

the requests module - 2

@app.route('/repos/<username>')
def repos(username):
    r = requests.get(f'https://api.github.com/users/{username}/repos')
    return render_template('repos.html', repos=r.json())

<ul>
    {% for repo in repos %}
        <li>{{ repo.name }}</li>
    {% endfor %}
</ul>

calling other routes from the template

<form action="/repos" method="get">
    <input type="text" name="username">
    <input type="submit" value="Submit">
</form>