Python - list of function/argument tuples

Question

def f1(n): #accepts one argument
    pass

def f2(): #accepts no arguments
    pass

FUNCTION_LIST = [(f1,(2)), #each list entry is a tuple containing a function object and a tuple of arguments
                 (f1,(6)),
                 (f2,())]

for f, arg in FUNCTION_LIST:
    f(arg)

The third time round in the loop, it attempts to pass an empty tuple of arguments to a function that accepts no arguments. It gives the error TypeError: f2() takes no arguments (1 given). The first two function calls work correctly - the content of the tuple gets passed, not the tuple itself.

Getting rid of the empty tuple of arguments in the offending list entry doesn't solve the problem:

FUNCTION_LIST[2] = (f2,)
for f,arg in FUNCTION_LIST:
    f(arg)

results in ValueError: need more than 1 value to unpack.

I've also tried iterating over the index rather then the list elements.

for n in range(len(FUNCTION_LIST)):
    FUNCTION_LIST[n][0](FUNCTION_LIST[n][1])

This gives the same TypeError in the first case, and IndexError: tuple index out of range when the third entry of the list is (f2,).

Finally, asterisk notation doesn't work either. This time it errors on the call to f1:

for f,args in FUNCTION_LIST:
    f(*args)

gives TypeError: f1() argument after * must be a sequence, not int.

I've run out of things to try. I still think the first one ought to work. Can anyone point me in the right direction?

Sven Marnach · Accepted Answer · 2012-08-09 12:23:42Z

9

Your comment in this code snippet shows a misconception relevant in this context:

FUNCTION_LIST = [(f1,(2)), #each list entry is a tuple containing a function object and a tuple of arguments
                 (f1,(6)),
                 (f2,())]

The expressions (2) and (6) are not tuples – they are integers. You should use (2,) and (6,) to denote the single-element tuples you want. After fixing this, your loop code should look thus:

for f, args in FUNCTION_LIST:
    f(*args)

See Unpacking Argument Lists in the Python tutorial for an explanation of the *args syntax.

answered Aug 9, 2012 at 12:23

Sven Marnach

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3 Comments

Benjamin Hodgson Over a year ago

Thanks for the great answer. Corollary question: is it possible to perform operations on the return value of the functions? Say I wanted to double the first entry: FUNCTION_LIST = [(f1*2,(2,)), (f1,(6,)),(f2,())] wouldn't work, would it, because you'd be trying to apply the multiplication operator to a function object, not the value returned by the function.

Benjamin Hodgson Over a year ago

Relatedly: is it possible to call other functions in the arguments tuple, which would be evaluated when the function is called? FUNCTION_LIST = [(f1,(time.time(),)), (f1,(6,)),(f2,())] wouldn't give me something time-dependent, because the call to time.time() would be evaluated when the list is populated.

Sven Marnach Over a year ago

@poorsod: For delayed execution, you should define functions doing what you want. For example def double_f1(x): return 2 * f1(x) or def f1_time(): return f1(time.time). In some cases, you can get away with lambda functions, but I'd recommend to use the sparingly.

Rostyslav Dzinko · Accepted Answer · 2012-08-09 12:24:21Z

3

The problem is that such notation:

(6)

evaluates to integer value and you need tuple, so write this way:

(6, )

and your asterisk notation will succeed.

answered Aug 9, 2012 at 12:24

Rostyslav Dzinko

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Comments

Lanaru · Accepted Answer · 2012-08-09 12:23:25Z

0

Try passing *() instead of (). The * symbol tells python to unpack the iterable that follows it, so it unpacks the empty tuple and passes nothing to the function, since the tuple was empty.

answered Aug 9, 2012 at 12:23

Lanaru

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Comments

Benjamin Hodgson · Accepted Answer · 2012-08-10 14:27:02Z

0

For the record, a nice alternative I have since discovered is the use of functools.partial. The following code does what I was trying to do:

from functools import partial

def f1(n): #accepts one argument
    pass

def f2(): #accepts no arguments
    pass

FUNCTION_LIST = [partial(f1,2), #each list entry is a callable with the argument pre-ordained
                 partial(f1,6),
                 partial(f2)] #the call to partial is not really necessary for the entry with no arguments.

for f in FUNCTION_LIST: f()

answered Aug 10, 2012 at 14:27

Benjamin Hodgson

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