How can I accept default parameter in function when 'None' or no argument is passed?

Question

I am writing a new python class with a generic function. At one point I have a requirement as follows

def function(a=1):
    ....
    ....
    print a # here I want a to be 1 if None or nothing is passed

Eg:

• a(None) should print 1
• a() should print 1
• a(2) should print 2

Is there a way to do this?

Hadrian

Answer 1

You can define a function with a default argument, later you can check for the value passed and print it:

def fun(a=1):
    a = 1 if a is None else a
    print(a)

Holster

Answer 2

def function(a=1):
    if a is None:
        a = 1

Contemporary

Answer 3

Should use None as default parameter.

def function(a=None):
    if a == None:
        a = 1
    print a

Otherwise you'll run into problems when calling the function multiple times.

Heald

What do you mean by "calling the function multiple times"? What kind of problems? – Gilthead

I suppose the problem mostly arises with mutable data types. Here's a link that explains it nicely: link – Heald

Answer 4

>>> def function(a=1):
...     print(a if a else 1)
... 
>>> function(None)
1
>>> function(2)
2
>>> function(1)
1
>>> function(123)
123
>>> function(0)
1

Anglicism

Answer 5

def function(a=1):
    print(1 if a is None else a)

function()
function(None)
function(2)
function(0)

Output:

1
1
2
0

Marasco

Answer 6

The default value you specify is going to be used only if you do not pass the value when calling the function at all. If you specified it as a=1 then calling it with zero arguments would indeed use 1 as the default value, but calling it with None will just put None to a.

When I want to give the None and "not specified at all" cases the same meaning, I do this:

def function(a=None):
    if a is None:
        a = 1
    ...

This way it will work as you want. Also, compared to the other solutions like

def function(a=1):
    if a is None:
        a = 1
    ...

you specify the "default" value (the 1) only once.

Gwennie