Jeff Peters wrote:
> Hi;
>
> I am trying to run a function inside a continuing loop, but do not seem
> to be able to pass any parameters (arguments ) when I do so.
> I have placed working and non-working code , with output below.
>
> ## This works:
>
> def loop(fn ):
> for i in range(5):
> fn( )
>
> def this_function(a=" i am not a string"):
> print( a )
>
> loop(this_function)
>
> ## with output:
> >>>
> i am not a string
> i am not a string
> i am not a string
> i am not a string
> i am not a string
> >>>
>
> ## But , this does not :
>
> def loop(fn ):
> for i in range(5):
> fn( )
>
> def this_function(a=" i am not a string"):
> print( a )
>
> loop(this_function("I am a string") ) ## note the only change is here
You are calling this_function() and then pass the result of the function
call to your other function loop().
Instead you need another function that builds a function that calls
this_function() with the desired argument:
>>> def loop(f):
... for i in range(5):
... f()
...
>>> def this_function(a):
... print(a)
...
>>> def make_function(f, arg):
... def g():
... f(arg)
... return g
...
>>> loop(make_function(this_function, "foo"))
foo
foo
foo
foo
foo
>>> loop(make_function(this_function, "bar"))
bar
bar
bar
bar
bar
>>>
Of course you could also change loop() to pass on arbitrary arguments:
>>> def loop(f, *args, **kw):
... for i in range(3):
... f(*args, **kw)
...
>>> loop(print, 1, 2)
1 2
1 2
1 2
>>> loop(print, 1, 2, sep="<-->")
1<-->2
1<-->2
1<-->2
Because building a function that just calls another function with some
predefined arguments is a common need the standard library has
functools.partial():
>>> from functools import partial
>>> print42 = partial(print, 42)
>>> print42()
42
>>> loop(print42)
42
42
42
Another variant is a lambda function with a default argument:
>>> loop(lambda a="whatever": print(a))
whatever
whatever
whatever
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