Question

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument

from random import random

def printNumber(num=random()):
    print num

for i in range(10):
    printNumber()

I want to see ten random numbers, but the ten random numbers are all the same!

Answer 1

Because the default arguments are only executed once, when the function is compiled.

From the Docs:

Default parameter values are evaluated 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. T

you can try something like this:

In [10]: def printNumber(num=None):
    return num if num is not None else random()
   ....: 

In [12]: printNumber()
Out[12]: 0.9620725546432438

In [13]: printNumber()
Out[13]: 0.8188258892156928

In [15]: printNumber(10)
Out[15]: 10