CSC401: Functions

A Simple Function

# define function
def ave(x, y):
    return (x + y) / 2.0

# use function
print ave(20, 30)
25.0

The Rules

Define a new function using def

Really creating a function object, then assigning it to a variable

Argument names follow in parentheses

No types

Finish at any time with return

Functions without return statements return None

def double(x):
    return x * 2
print double(2.5)
5.0
print double
<function double at 0xa0c78b4>

Scope

Functions created in a block are local to it

x = 123
def f(arg):
    x = arg
    def g():
        x = 2
        y = 1
        print "x in g is ", x
        print "y in g is ", y
    g()
    print "x in f is ", x
    print "y in f is ", y
f(999)

x in g is 2
y in g is 1
x in f is 999
y in f is NameError: global name 'y' is not defined

print x
123

Memory

Function arguments always copied

Which means structures are aliased

Just as in Java

def mutate(x, y):
    x = 0
    y[0] = 0
a = 1
b = [1, 1, 1]
mutate(a, b)
print a, b
1, [0, 1, 1]

Default Argument Values

Can provide defaults for arguments

Arguments without defaults must come first

def withTax(val, percent=15):
    return val * (1.0 + percent/100.0)
print withTax(10.00)       # default
print withTax(10.00, 7)    # explicit
11.5
10.7

Named Arguments

Can pass arguments in any order using names

def show(first, second):
    print first, second
show(1, 2)
1 2
show(second=9, first=0)
0 9

Mixing Defaults and Names

Matching rules are intuitive

Match left-to-right

Unnamed must precede named

def show(first, second, third=3):
    print first, second, third
show(second=9, first=0)
show(1, 2, 5)
0 9 3
1 2 5

Extra Arguments

Any extra unnamed arguments put in a tuple called *extra

A tuple is like a constant list

Uses () instead of []

Exercise for the reader: look up tuples in Python documentation

Only the * matters: could call the argument *fred

If no * argument present, extra values are illegal

def show(first, *extra):
    print first, extra
show(10, 20, 30, 40)
10 (20, 30, 40)

Extra Named Arguments

Any extra named arguments put in a dictionary called **named

Again, only the ** is important

def user(id, **named):
    print id, "=", named
user(0, name="Greg", age=40)
user(1, hair="some", eyes=2)
0 = {'age' : 40, 'name' : 'Greg'}
1 = {'hair' : 'some', 'eyes' : 2}

Extra Argument Example

An Interesting Example

Who said Python is simple and clean? :)

x = 1
def double(x=x):
    return x * 2
y = 4
print double(y)
8
print double()
2

Slides originally created by Greg Wilson. Initial adaptation for CSC401 by David James. Revisions by Michelle Craig, Michael Szamosi, Karen Reid, and David James. Revisions for CSC401 Winter 2006 by Cosmin Munteanu.