Passing arguments

DEFAULT ARGUMENTS

Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call.

def print_word(word, n=1):
    for i in range(n):
        print("{} {}".format(i, word))

print_word("spam")   # default n=1
print_word("spam", 3)   # n=3
print_word(n=4, word="spam")   # using keywords 'n' and 'word'

# Problems with mutable default arguments.

def tester1(x, L=[]):   # L is created once
    L.append(x)
    print("tester1 {}".format(L))

def tester2(x, L=None):
    if L is None:   # explicit test for None
        L = []
    L.append(x)
    print("tester2 {}".format(L))

tester1(1)   # [1]
tester1(2)   # [1, 2]
tester1(3)   # [1, 2, 3]
tester2(1)   # [1]
tester2(2)   # [2]
tester2(3)   # [3]
# Using an external list.
M = []
tester1(5, M)
assert M == [5]
tester2(6, M)
assert M == [5, 6]

EXCESS POSITIONAL ARGUMENTS

# '*identifier' is initialized to a tuple with excess positional arguments

def unique_letters(*arguments):
    letters = set()
    for word in arguments:   # 'arguments' is a tuple
        letters.update(word)
    return letters

unique_letters("abc", "cde", "efg")   # set(['a', 'e', 'f', 'b', 'c', 'g', 'd'])

KEYWORD ARGUMENTS

# '**identifier' is initialized to a new ordered mapping receiving
# any excess keyword arguments

def university(**keywords):
    for key in sorted(keywords):   # keywords is a dict
        print("{} : {}".format(key, keywords[key]))

university(rektor="Jacek Popiel", kanclerz="Monika Harpula")
# kanclerz : Monika Harpula
# rektor : Jacek Popiel

EXAMPLE

def func(a1, a2=v2, *a3, **a4): pass

# a function call is func(x1, x2, x3, x4, b1=x5, b2=x6)
# a1=x1, positional argument
# a2=x2, positional argument (default value will not be used)
# a3=(x3, x4), excess positional arguments
# a4={"b1":x5, "b2":x6}, excess keyword arguments

# The most general form is
def general_func(*arguments, **keywords): pass

UNPACKING ARGUMENTS

args = (1, 5)
range(*args)   # range(1, 5)

def describe(name, age=0, position=None):
    print("{} age {} position {}".format(name, age, position))

person = {"name": "Bogdan", "age": 30, "position": "teacher"}

describe("Adam", 20, "student")
# Adam age 20 position student

describe(**person)
#describe(name="Bogdan", age=30, position="teacher")   # the same
# Bogdan age 30 position teacher

FORCING ARGUMENTS

def func(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2): pass

def func_pos_only(arg, /): pass   # func_pos_only(25)

def func_kwd_only(*, arg): pass   # func_kwd_only(arg=34)

# Exercise: help(sorted)
sorted(iterable, /, *, key=None, reverse=False)
# Exercise: help(abs)
abs(x, /)