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 ```

​

• 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])
    

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