How to bind a constant parameter in Python?

Question

I have got a class similar to

class C:
    def __init__(self, a):
        self.a = a
    def noParam(self):
        return self.a
    def withParam(self, b)
        return self.a + b
instC = C(5.)

I need to pass a method of a particular instance of a class as a parameter. Passing instC.noParam works fine, but how do I pass instC.withParam with b always equal to say 239? Thanks.

Nick Bastin · Accepted Answer · 2013-04-17T17:47:24.353

12

You want to use functools.partial:

import functools

instC = C(5.)
meth = functools.partial(instC.withParam, 239)

Then you can pass this method (bound with both your instance and the value 239):

do_thing(meth)

edited Apr 17 '13 at 17:47

answered Apr 17 '13 at 17:44

Nick Bastin

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2

This is the best approach, but would also be good to mention `lambda` -- it is both an alternative and also helps explain what `partial` is doing. – Francis Avila Apr 17 '13 at 18:11
@Francis: why is this one better than lambda? – Yulia V Apr 18 '13 at 09:29
sorry should have googled first (http://stackoverflow.com/questions/3252228/python-why-is-functools-partial-necessary) – Yulia V Apr 18 '13 at 11:03
1

I prefer a lambda to `functools.partial` because a lambda is easier to debug. – Peter Graham Apr 21 '13 at 23:16

aldeb · Answer 2 · 2013-06-02T14:49:20.873

7

You can use lambda functions to bind parameters:

meth = lambda: instC.withParam(239)

edited Jun 02 '13 at 14:49

answered Apr 17 '13 at 17:45

aldeb

6,588
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How to bind a constant parameter in Python?

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