Assign pass to a function in Python

Question

I have a piece of code that defines a function that takes a function as an argument, like so:

def stuff(n, f):
  f(n)

Now, I want to provide some default value of f that does nothing. So I figured I'd use pass, like so:

def stuff(n, f = None):
  if(f is None):
    f = pass
  f(n)

But this does not compile. How should I be doing this?

About the formatting: do not leave spaces around the `=` in the signature of a function. — Björn Pollex, Feb 28 '11 at 10:14
While we're nitting the formatting, don't use less than 4 spaces for indentation--2 spaces is essentially unreadable--and don't put parentheses around conditionals as if you're writing C. — Glenn Maynard, Feb 28 '11 at 11:41
What everybody is saying is that you should read the [PEP 8](http://www.python.org/dev/peps/pep-0008/) — Fábio Diniz, Feb 28 '11 at 12:20

score 19 · Accepted Answer · edited Feb 28 '11 at 10:15

19

The pass is a keyword for the interpreter, a place holder for otherwise nothing. It's not an object that you can assign. You can use a no-op lambda.

f = lambda x: None

edited Feb 28 '11 at 10:15

Björn Pollex

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answered Feb 28 '11 at 10:08

Keith

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OT, but why would the designers need to create a new keyword just for a noop? bizarre – jberryman Jan 01 '13 at 22:15
1

@jberryman because Python doesn't have block delimiters. In languages that do you would typically write `{}`. But in Python it would be... nothing. So `pass` is there to make it syntactically complete. – Keith Jan 01 '13 at 22:26
@jberryman Where? `pass` is just a keyword to act as a placeholder for otherwise empty block. It probably makes parsing easier. It's not an object. None is an object that means no other value. They have different semantics. – Keith Jan 02 '13 at 02:02

Nicolas Lefebvre · Answer 2 · 2011-02-28T10:48:50.357

9

Why not simply this ?

def stuff(n, f=None):
    if f is None:
        return
    return f(n)

edited Feb 28 '11 at 10:48

answered Feb 28 '11 at 10:07

Nicolas Lefebvre

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2

Minor improvement: [PEP8](http://www.python.org/dev/peps/pep-0008/) advises to use `is None` instead of `== None` ([rationale](http://jaredgrubb.blogspot.com/2009/04/python-is-none-vs-none.html)). – Oben Sonne Feb 28 '11 at 10:34
Thanks, I'd wondered which was best but never thought to check ! Answer fixed accordingly. – Nicolas Lefebvre Feb 28 '11 at 10:48
+1 Much better than the other solutions since it clearly states the intent. – Aaron Digulla Feb 28 '11 at 11:13
As far as this question has a "correct" answer, this is it. The other answers all needlessly obfuscate such a trivial function. – Glenn Maynard Feb 28 '11 at 11:39
If the OP wants to pass the function object on to another function, this approach does not work. – Björn Pollex Feb 28 '11 at 12:32
Man, I'd love to give you the answer...while this does answer exactly how I described the issue, my description was oversimplified...in my program, stuff does more than call f(n) and f(n) is called multiple times. If I could pick two answers, though,I'd pick this. – LinuxMercedes Mar 01 '11 at 09:14
No problem ! I as Space_C0wb0y pointed out, the correct answer depended on your exact scenario so I figured you'd pick whichever suited best :) – Nicolas Lefebvre Mar 01 '11 at 09:19

Björn Pollex · Answer 3 · 2011-02-28T10:22:03.230

5

A little shorter:

def stuff(n, f=None):
  f = f or (lambda x: None)
  f(n)

Or even better:

def stuff(n, f=None):
    f(n) if f is not None else None

edited Feb 28 '11 at 10:22

answered Feb 28 '11 at 10:10

Björn Pollex

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Is your first version Python3K ? That syntax does not appear to be valid for python 2.7. – Nicolas Lefebvre Feb 28 '11 at 10:16
@Bethor: Thanks, fixed. Tested on 2.6. – Björn Pollex Feb 28 '11 at 10:22
or even simpler: `def stuff(n, f = None): f and f(n)` – codecraft Feb 28 '11 at 11:09
@codecraft: Indeed. But I would argue that in this case clarity suffers needlessly. – Björn Pollex Feb 28 '11 at 11:11
agreed. Using logical operators for control flow makes things really perl-like and is probably not very pythonic – codecraft Feb 28 '11 at 11:15
Ick: don't use the ternary operator as flow control! This answer is encouraging optimizing for line count instead of, and at the expense of, clarity. – Glenn Maynard Feb 28 '11 at 11:36
@Glenn: I agree that my answer is not good for the case where the function has not return value, and only the side effect counts. But, would you agree that this would be fine if the focus was on the return value? – Björn Pollex Feb 28 '11 at 12:20
I don't see any advantage to this over Bethor's answer. – Glenn Maynard Feb 28 '11 at 15:57
@Glenn: I have been thinking, and I agree that Bethor's answer is better, except for the case when the function object itself will be passed as argument to other functions. – Björn Pollex Feb 28 '11 at 16:00

score 4 · Answer 4 · answered Feb 28 '11 at 09:59

4

You can't assing empty. But you can creaty internal function, that you will assign to f.

def stuff(n, f=None):
  def empty(n):
    pass
  if f is None:
    f = empty
  f(n)

answered Feb 28 '11 at 09:59

gruszczy

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score 3 · Answer 5 · answered Sep 22 '14 at 10:30

3

The generic noop function :

f = lambda *x,**y:None

You can call

f()
f(0,foo=0)

answered Sep 22 '14 at 10:30

Alcolo47

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score 1 · Answer 6 · answered Jun 02 '16 at 07:35

1

I'd go with a

def nop(*args, **kwargs):
    pass

def algo(func=nop):
    do_something()
    func()
    do_something_else()

algo() # nop is called
algo(some_callback) # some_callback is called

IMHO looks cleaner than

if foo:
    foo()

answered Jun 02 '16 at 07:35

Ivan Dives

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Assign pass to a function in Python

6 Answers6