Python - Introduction

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This post covers Introduction to Python Programming.

Hello Python!

  • Assignment Operator, =

    • No need to declare before assignment
  • Function call - print()

  • Comment - single line comment or triple-quoted string literals

  • if statement

  • string - single quote or double quote

  • type

  • Addition, Subtraction, Multiplication, True Division (/); Floor Division (//); Modulus (%); Exponentiation (**); Negation

  • Order - Parentheses, Exponents, Multiplication/Division, Addition/Subtraction

  • Built-in Functions: min, max, abs, float, int

  • help() function e.g. help(min)

  • Defining Function

    • Docstring - triple quoted string for help()
    • return is optional (if no return then python return None): ret = print()
    • default arguments
  • Function applied to Function - Higher Order Function

    • def pow2(x):
          return x*x
          
      def call_pow2(fn, arg):
          val = fn(arg)
          return val
          
      print(pow2(3))  # 9
      print(call_pow2(pow2, 6))  # 36
          
      def pow4(fn, arg):
          val = fn(arg)
          val = fn(val)
          return val
          
      print(pow4(pow2, 2)) # 16
      
    • # Which number is biggest modulo 5
      print(max(120, 102, 104)) # 120
          
      def mod5(x):
          return x % 5
          
      print(max(120, 102, 104, key=mod5)) # 104
      
  • Data Type: bool

    • # True or False
      # compbine: and or not
      3.0 == 3 # True
      '3' == 3 # False
      # if elif else
      bool(1) # True for all numbers except 0
      bool(0) # False
          
      bool('abc') # True for all strings/other types except empty
      bool('') # False
      x = 5
      if x:
        print('Greater than zero or non-empty string')
      else:
        print('equal to zero or empty string')
      
  • Lists

    • Ordered sequence of values

    • nums = [2, 4, 6, 8]
          
      nums = [[1, 3, 5, 7], [2, 4, 6, 8]]
          
      nums = [[1, 3, 5, 7], 
              [2, 4, 6, 8]]
          
      nums = [[1, 3, 5, 7], 
              [2, 4, 6, 8],]
          
      nums = [['1', 3, 5, '7'], 
              [2, '4', '6', 8]]
          
      # Indexing
      nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
      nums[0], nums[-1]
          
      # Slicing
      num[1:5], num[3:], nums[-3:]
          
      # List are mutable (can be modifiied in place)
      nums = [1, 3, 5, 7, 9]
      nums[-2:] = [11, 13, 15] # [1, 3, 5, 11, 13, 15]
          
      # List functions
      len(), sum(), sorted(), min(), max()
      
    • Objects

      • Everything in python is an object

      • Object carry somethings (attributes and methods) around with them and can be accessed using dot

      • num = 5
        print(num.imag) # 0
              
        num = 5 +2j
        print(num) # (5+2j)
        print(num.imag) # 2.0
              
        num = 100
        print(num.bit_length()) # only for int
              
        

        List Methods: append, pop, search(index), in

        nums = [1, 3, 5, 7] nums.append(9) help(nums.append)

        nums.pop()

        nums.index(5)

        9 in nums

        help(nums) # see all methods for nums ```

‚Äč

  • Tuples

    • t = (1, 2, 3)
            
      # Immutable
      t[0] = 10 # Error
            
      # Used to return multiple values
      num = 0.25
      num.as_integer_ratio() # return tuple (1, 4)
            
      # Swap values
      a = 2
      b = 5
      a, b = b, a
      
  • Loops

    • planets = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']
            
      for planet in planets:
          print(planet, end=' ')
            
      print()
      for idx in range(len(planets)):
            print(planets[idx], end=' ')
            
      print()
      i = 0
      while i <= 7:
          print(planets[i], end=' ')
          i += 1
      
  • List Comprehensions

    • planets = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']
            
      lengths = [planet for planet in planets if len(planet) < 6]
      print(lengths)
            
      lengths = [planet if len(planet) < 6 else 'None' for planet in planets]
      print(lengths)
      
  • Strings

    • hello = "hello\nworld"
      print(hello)
            
      print()
      hello = """hello
            
      world"""
      print(hello)
            
      # Strings are sequence and can be sliced
      hello[0], hello[-5:]
            
      # String is Immutable
      hello[0] = 'H' # error
      
    • # String Methods
      hello = "hello world"
      hello.upper()
      hello.lower()
      hello.index('world')
      hello.startswith('hello')
      hello.endswith('world')
      words = hello.split()
      hello = " ".join(words)
            
      datestr = '2021-01-11'
      year, month, day = datestr.split('-')
      '/'.join([year, month, day])
            
      date = f'{year}/{month}/{day}'
      
  • Dictionary

    • planets = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']
      p2i = {planets[idx]:idx for idx in range(len(planets))}
      i2p = { v:k for k,v in p2i.items()}
      
  • External Libraries

    • import math
      print(type(math))
      print(dir(math))
      print(math.pi)
      print(math.log(8, 2))
      print(help(math.log))
      print(help(math))
            
      import math as mt
      print(mt.pi)
            
      from math import pi, log
      print(pi)
      print(log(8, 2))
      
    • import numpy
      print(type(numpy))
      print(type(numpy.random))
      print(dir(numpy.random))
            
      rolls = numpy.random.randint(low=1, high=6, size=10)
      print(rolls)
            
      print(type(rolls)) # What is this
      print(dir(rolls)) # What can be done with it
            
      print(rolls.mean())
      print(rolls.tolist())
      
    • # Opertaor Overloading - depends on the library
      # [1, ,2, 3, 4] + 5 # Error in the list
            
      import numpy as np
      np.array([1, 2, 3, 4]) + 5 # works fine in array
            
      print(np.array([1, 2, 3, 4, 5]) <= 3)
            
      list_ = [[1, 2, 3], [4, 5, 6]]
      list_ = np.array(list_)
      print(list_)
      print(list_[1, -1])