Python: Function

By Xah Lee. Date: . Last updated: .

Here's a example of defining a function.

# python 3

def ff(x, y):
    """ff(x, y) returns x+y."""
    return x+y

print(ff(3,4))
# 7

A string immediately following the function def line is the function's documentation.

return is optional. If a function does not return statement, it returns None.

Unspecified Number of Positional Parameters

To define unspecified number of positional parameters, use *name as last item. Your function will receive it as a tuple.

# python 3

# function with unspecified number of args
def ff(*x):
    # x is received as a tuple.
    return x

rr = ff(5,6)

print(rr)
# (5, 6)

print(type(rr))
# <class 'tuple'>

Named Parameters (Keyword Arguments)

A function can have Named Parameters, called Keyword Argument in Python.

Python's “keyword argument” are automatically optional. If no argument is given in a function call, a default value is used.

# python 3

# function with keyword parameters.

# Keyword parameters are automatically optional, with default values.
# They must come after positional parameters, if any
def ff(x, y=100, z=200):
    return "x ⇒ {}, y ⇒ {}, z ⇒ {}".format(x, y, z)

print(ff(3))
# x ⇒ 3, y ⇒ 100, z ⇒ 200

# optional parameter name can be omitted. If so, they go by order.
print(ff(3, 2))
# x ⇒ 3, y ⇒ 2, z ⇒ 200

print(ff(3, 5, 6))
# x ⇒ 3, y ⇒ 5, z ⇒ 6

# keyword argument must come after positional arguments
print(ff(3, y=2))
# x ⇒ 3, y ⇒ 2, z ⇒ 200

print(ff(3, z=2))
# x ⇒ 3, y ⇒ 100, z ⇒ 2

# keyword arguments can be any order
print(ff(3, z=8, y=9))
# x ⇒ 3, y ⇒ 9, z ⇒ 8

Unspecified Number of Unnamed Keyword Parameters

For unspecified number of unnamed keyword parameters. You need to put a **name as last item in your parameter list in definition. Your function will receive it as a dictionary.

# python 3

# unspecified number of keyword args
def ff(**dd):
    return dd

rr = ff( z = 8, c = 7, aa = 4, b = 3)

print(rr)
# {'z': 8, 'c': 7, 'aa': 4, 'b': 3}

print(type(rr))
# <class 'dict'>

The **name can be used with normal paramaters. But it must be the last.

# python 3

def ff(a, b=1, c=1, **xyz):
    # xyz is a dictionary
    print("a is: ", a)
    print("b is: ", b)
    print("c is: ", c)
    print("xyz is: ", xyz)

# note, no b
ff(3, rabbit = 7, cat = "a", tiger = 33, c = 8)

# a is:  3
# b is:  1
# c is:  8
# xyz is:  {'rabbit': 7, 'cat': 'a', 'tiger': 33}

The positional parameters, and *tup tuple and **dict dictionary can be used together, but tuple must come first.

# python 3

def ff(a, *mmm, **zzz):
    # mmm is a tuple
    # zzz is a dictionary
    print("a is:", a)
    print("mmm is:", mmm)
    print("zzz is:", zzz)

ff(3, 4, 5, 6, rabbit = 7, cat = 9, tiger = 33)

# a is: 3
# mmm is: (4, 5, 6)
# zzz is: {'rabbit': 7, 'cat': 9, 'tiger': 33}

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Python

  1. Python 3 Basics
  2. Python 2 Basics
  3. Python 2 and 3 Difference
  4. Print Version
  5. Builtin Help
  6. Quote String
  7. String Methods
  8. Format String
  9. Operators
  10. Complex Numbers
  11. True, False
  12. if then else
  13. Loop
  14. List Basics
  15. Loop Thru List
  16. Map f to List
  17. Copy Nested List
  18. List Comprehension
  19. List Methods
  20. Sort
  21. Dictionary
  22. Loop Thru Dict
  23. Dict Methods
  24. Tuple
  25. Sets
  26. Function
  27. Closure
  28. 2 Closure
  29. Decorator
  30. Class
  31. Object, ID, Type
  32. List Modules
  33. Write a Module
  34. Unicode 🐍

Regex

  1. Regex Basics
  2. Regex Reference

Text Processing

  1. Read/Write File
  2. Traverse Directory
  3. File Path
  4. Process Unicode
  5. Convert File Encoding
  6. Find Replace in dir
  7. Find Replace by Regex
  8. Count Word Frequency

Web

  1. Send Email
  2. GET Web Page
  3. Web Crawler
  4. HTTP POST

Misc

  1. JSON
  2. Find Script Path
  3. Get Env Var
  4. System Call
  5. Decompress Gzip
  6. Append String in Loop
  7. Timing f timeit
  8. Keyword Arg Default Value Unstable
  9. Check Page Load Size
  10. Thumbnail Generation