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Functions

Built-in functions

Python provides a bunch of built-in functions

s = [1,2,3]
print(len(s))  # 3

from math import sqrt
print(sqrt(4)) # 2.0

User defined functions

Functions are defined in Python using def keyword.

def sum(a,b):
  print(a+b)
sum(3,5) # 8

Functions can return multiple values.

def square_cube(a: int): # this is called type hinting. this does not enforce type of parameter
  return pow(a,2), pow(a, 3)
s,c = square_cube(3)
print(s,c) # 9 27

Parameters of a function can have default values. If suppose 3rd parameter of function which takes 5 parameters has a default value, then 4th and 5th parameter should also have default values. Arguments can be passes in dictionary format in a function call. This feature is useful when arguments for all parameters is not required in function call.

def product(a, b=7, c=10):
  return a*b*c
print(product(1,2,3)) # 6
print(product(1,b=5)) # 50
print(product(a=3, c=3)) # 63

You must have noticed that to call a function, round brackets are required, after the function name, even if no parameters are to be passed. Sequence of function call matters.

def func1():
    return func2()
def func2():
    print("Hola")
func1() # this works
def func3():
    return func4()
func3() # this does not work
def func4():
    print("There")
print(type(func4)) # function

d = func3
d() # same as func3

Function Arguments and Mutability

Immutable objects are safe from unintended side-effects. Lets say, a function takes an immutable object as parameter. When function is called, the arguments are passed by reference. Hence, the original value does not suffer from the possibilty of being modified. However, if the parameters are mutable, the original data is changed.

def change_string(a):
    a = a + "hello"
    return a

def change_list(l):
    l.append(100)

s = "world"
print("before: s:", s)  # world
print("function called: ", change_string(s))  # worldhello
print("after: s:", s)  # world
l = [1, 2, 3]
print("before: l:", l)  # [1, 2, 3]
print("function called:", change_list(l))
print("after: l:", l)  # [1, 2, 3, 100]

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