PYTHON 01: Functions

• Eliminate duplicate codes
• Re-usable

• How to define function:

  def func_name(param1, param2):
    return param1 + param2
  

  • Parameter == argument
  • Function is a code block -> need indentation
  • Function name and parameter name must be meaningul

• Default parameter

  • If not specified on usage, pass default value in the function

  def add(x, y, z=1):
    a = x + y + z
    return a
  
  add(10, 6) # 17
  

  • Cannot be located in middle or start

    SyntaxError: non-default argument follows default argumnet

  def good1(a, b, c=2):
  def good2(a, b=1, c=2):
  
  def bad1(a, b=1, c):
  def bad2(a=1, b=2, c):
  

• Keyword parameter

  • Can explicitly tell parameter name when passing
  • Therefore, parameter order does not matter
  • Rarely used… if used, use it every where, else, do not use it for clean coding

  add(y=5, x=10, z=1) # 16
  

• Return

  • End of function
  • If no value assigned on return or does not use return keyword in function, the function returns None
  • Can have multiple return: Return values as tuple

  def add_mul(x, y):
    s = x + y
    m = x * y
    return s, m
  
  c = add_mul(20, 3)
  print(c) # (23, 60)
  a, b = add_mul(20, 3) # Can use tuple unpacking
  

• Variable scope

  • Global variable keeps alive until the program exit
  • Local variable (variable inside code block) disappears as soon as the block finishes its job
  • Local variable has priority if global variable has same name

• Variable length argument (가변길이 인자)

  • Acknowledge as tuple
  • Number of param passed is not fixed

    e.g.) print(), format(), …

  def test(*args):
    print(type(args))
  
  test(10, 20, 30) # <class 'tuple'>
  test() # acknowledge as empty tuple
  

  • Traditionally, use *args, **kwargs as a name for variable length arguments

• Keyword paramenter

  • Acknowledge as dictionary

  def test(**kwargs):
    print(type(kwargs))
    for k, v in kwargs.items():
      print(k, v)
  
  test(a=10, b=20, name='Shaun') # <class 'dict'>
  # a 10
  # b 20
  # name Shaun
  

  • Using {} placeholder and .format()

  asd = 'Hi {}, I am Mee{}!'
  print(asd) # Hi {}, I am Mee{}!
  asd = 'Hi {}, I am Mee{}!'.format('Shaun', 6)
  print(asd) # Hi Shaun, I am Mee6!
  asd = 'Hi {name}, I am Mee{number}!'.format(name = 'Shaun', number = 6)
  print(asd) # Hi Shaun, I am Mee6!
  

  For more info about .format() method: https://pyformat.info/

• Lambda

  • A nameless function w/ only one line
  • Good for one-time usage function

  def square(param):
    return x**2
  square(5) # 25
  
  square2 = lambda param:returnValue**2 # input:output
  square2(5) # 25
  
  mult = lambda x,y:x*y
  mult(2,3) # 6
  

  e.g.) .sort(), filter(), map(), reduce()

  def str_len(s):
    return len(s)
  
  strings = ['bnm', 'asd', 'cvb']
  strings.sort(key=str_len) # key's input => LOCAL function
  strings.sort(key=lambda s:len(s))
  print(strings)
  
  # filter(function, list)
  nums = [1, 2, 3, 4, 5, 6]
  list(filter(lambda n:n%2==0, nums)) # 2, 4, 6
  
  # map(function, list)
  list(map(lambda n:n**2, nums)) # 1, 4, 9, ...
  list(map(lambda n:n**2==0, nums)) # False, True, False, ...
  
  # reduce: consecutive operation of 2 previous elements
  import functools
  a = [1, 3, 5, 8]
  functools.reduce(lambda x,y:x+y, a) # 17