Python dynamic function generation

Question

I am using PyQt and want to create a menu based on a list of strings.

The problem is that when I want to call 'addAction', it requires a call-back function (for each string) that does not take any arguments.

For simple menus, this would be fine: e.g.

menu.addAction("Open", self.open)
menu.addAction("Exit", self.quit)

However, I want to just use a single function and have the 'action string' passed in as an argument.

I am wondering if python can do something like this:

def f(x, y):
    print x + 2*y



# These 2 variables are of type: <type 'function'>
callback1 = f
callback2 = f(x=7, *)
# NB: the line above is not valid python code.
# I'm just illustrating my desired functionality

print callback1(2,5)  # prints 12
print callback2(5)    # prints 17

Here is my code snippet:

def printParam(parameter):
    print "You selected %s" % parameter


# parameters = list of strings

menu_bar = QMenuBar()
menu = menu_bar.addMenu('Select Parameter')
for parameter in parameters:
    # This line will not work because the action does not pass in
    # any arguments to the function
    menu.addAction(parameter, printParam)

Any suggestions greatly appreciated

"Use a closure". A closure is simply a function that closes over a free variable in the enclosing scope. — user166390
– user166390, Commented Jul 13, 2011 at 17:03
@pst: No, that'd be reinventing the wheel given the presence of functools.partial. Plus "closure" is a much broader concept ;) — user395760
– user395760, Commented Jul 13, 2011 at 17:07
@delan :) What do you mean by closure being a broader concept? — user166390
– user166390, Commented Jul 13, 2011 at 19:07

unholysampler · Accepted Answer · 2011-07-13 17:04:46Z

6

functools.partial() allows you to supply some of the arguments ahead of time. It will allow you to make custom callbacks as you want.

>>> from functools import partial
>>> basetwo = partial(int, base=2)
>>> basetwo.__doc__ = 'Convert base 2 string to an int.'
>>> basetwo('10010')
18

answered Jul 13, 2011 at 17:04

unholysampler

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Comments

Thomas K · Accepted Answer · 2011-07-13 17:04:26Z

4

You can use a closure, like this:

def printParam(parameter):
    def callback():
        print "You selected %s" % parameter
    return callback

for parameter in parameters:
    menu.addAction(parameter, printParam(parameter))

answered Jul 13, 2011 at 17:04

Thomas K

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2 Comments

user395760 Over a year ago

As I wrote before, if you just need to apply a function partially, just using functools.partial is shorter and (arguably) simpler. A higher order function like this is a good choice if you need more complex logic though.

Thomas K Over a year ago

Yes, functools.partial does the job here. But there's something beautiful about closures.

user395760 · Accepted Answer · 2011-07-13 17:05:30Z

2

Yes, it's called partial application. The standard library has a class to apply a function partially, functools.partial. You'd write partial(f, x=7) or partial(f, 7) (since it's a positional argument and you don't need to skip over any positional arguments) in your example.

answered Jul 13, 2011 at 17:05

user395760

Comments

murgatroid99 · Accepted Answer · 2011-07-13 17:12:14Z

1

Use functools.partial to do the following (easier to demonstrate on your simple example):

import functools
callback2=functools.partial(f, 7)
callback3=functools.partial(f, y=5) #if you want to pass `y` instead of `x`

If you always want to pass a particular argument, then as others have pointed out you could use a closure:

def f(x):
    def g(y):
        print x + 2*y
    return g
callback1=f
callback2=f(7)
callback1(2)(5) #prints 12
callback2(5) #prints 17

edited Jul 13, 2011 at 17:12

answered Jul 13, 2011 at 17:04

murgatroid99

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2 Comments

Ismail Badawi Over a year ago

Note that with this, you have to write callback2(y=5), otherwise you get an error about receiving multiple values for x when you call callback2(5). Easier in this case to write functools.partial(f, 7)

murgatroid99 Over a year ago

@isbadawi Noted and fixed. Thanks.

phkahler · Accepted Answer · 2011-07-13 17:41:58Z

0

def my_menu_func(self, string):
   # whatever your function does with that parameter

then for setting up the menu:

x = "Open"
a = eval("lambda : my_menu_func('"+x+"')")
menu.addAction(x, a)

I tried using a lamba directly inside addAction, but that forms a closure containing the variable x, so any subsequent local changes to x would alter the parameter passed in the call to my_menu_func. Doing it with the eval you can iterate over a list of strings and create a whole menu for example by:

for x in List_of_Menu_Items:

do the same thing.

answered Jul 13, 2011 at 17:41

phkahler

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2 Comments

aukaost Over a year ago

Wasn't my vote but I'd guess it's because of the 'eval' part of your answer.

phkahler Over a year ago

But without the eval it creates a function that calls my_menu_func with a reference to the local variable x instead of the value "Open". So on subsequent calls, you don't get what you think.

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