I am using lambda functions for GUI programming with tkinter. Recently I got stuck when implementing buttons that open files:
self.file=""
button = Button(conf_f, text="Tools opt.",
command=lambda: tktb.helpers.openfile(self.file))
As you see, I want to define a file path that can be updated, and that is not known when creating the GUI. The issue I had is that earlier my code was :
button = Button(conf_f, text="Tools opt.",
command=lambda f=self.file: tktb.helpers.openfile(f))
The lambda function had a keyword argument to pass the file path. In this case, the parameter f was not updated when self.file was.
I got the keyword argument from a code snippet and I use it everywhere. Obviously I shouldn't...
This is still not clear to me... Could someone explain me the difference between the two lambda forms and when to use one an another?
PS: The following comment led me to the solution but I'd like a little more explanations: lambda working oddly with tkinter
I'll try to explain it more in depth.
If you do
i = 0
f = lambda: i
you create a function (lambda is essentially a function) which accesses its enclosing scope's i variable.
Internally, it does so by having a so-called closure which contains the i. It is, loosely spoken, a kind of pointer to the real variable which can hold different values at different points of time.
def a():
# first, yield a function to access i
yield lambda: i
# now, set i to different values successively
for i in range(100): yield
g = a() # create generator
f = next(g) # get the function
f() # -> error as i is not set yet
next(g)
f() # -> 0
next(g)
f() # -> 1
# and so on
f.func_closure # -> an object stemming from the local scope of a()
f.func_closure[0].cell_contents # -> the current value of this variable
Here, all values of i are - at their time - stored in that said closure. If the function f() needs them. it gets them from there.
You can see that difference on the disassembly listings:
These said functions a() and f() disassemble like this:
>>> dis.dis(a)
2 0 LOAD_CLOSURE 0 (i)
3 BUILD_TUPLE 1
6 LOAD_CONST 1 (<code object <lambda> at 0xb72ea650, file "<stdin>", line 2>)
9 MAKE_CLOSURE 0
12 YIELD_VALUE
13 POP_TOP
3 14 SETUP_LOOP 25 (to 42)
17 LOAD_GLOBAL 0 (range)
20 LOAD_CONST 2 (100)
23 CALL_FUNCTION 1
26 GET_ITER
>> 27 FOR_ITER 11 (to 41)
30 STORE_DEREF 0 (i)
33 LOAD_CONST 0 (None)
36 YIELD_VALUE
37 POP_TOP
38 JUMP_ABSOLUTE 27
>> 41 POP_BLOCK
>> 42 LOAD_CONST 0 (None)
45 RETURN_VALUE
>>> dis.dis(f)
2 0 LOAD_DEREF 0 (i)
3 RETURN_VALUE
Compare that to a function b() which looks like
>>> def b():
... for i in range(100): yield
>>> dis.dis(b)
2 0 SETUP_LOOP 25 (to 28)
3 LOAD_GLOBAL 0 (range)
6 LOAD_CONST 1 (100)
9 CALL_FUNCTION 1
12 GET_ITER
>> 13 FOR_ITER 11 (to 27)
16 STORE_FAST 0 (i)
19 LOAD_CONST 0 (None)
22 YIELD_VALUE
23 POP_TOP
24 JUMP_ABSOLUTE 13
>> 27 POP_BLOCK
>> 28 LOAD_CONST 0 (None)
31 RETURN_VALUE
The main difference in the loop is
>> 13 FOR_ITER 11 (to 27)
16 STORE_FAST 0 (i)
in b() vs.
>> 27 FOR_ITER 11 (to 41)
30 STORE_DEREF 0 (i)
in a(): the STORE_DEREF stores in a cell object (closure), while STORE_FAST uses a "normal" variable, which (probably) works a little bit faster.
The lambda as well makes a difference:
>>> dis.dis(lambda: i)
1 0 LOAD_GLOBAL 0 (i)
3 RETURN_VALUE
Here you have a LOAD_GLOBAL, while the one above uses LOAD_DEREF. The latter, as well, is for the closure.
I completely forgot about lambda i=i: i.
If you have the value as a default parameter, it finds its way into the function via a completely different path: the current value of i gets passed to the just created function via a default parameter:
>>> i = 42
>>> f = lambda i=i: i
>>> dis.dis(f)
1 0 LOAD_FAST 0 (i)
3 RETURN_VALUE
This way the function gets called as f(). It detects that there is a missing argument and fills the respective parameter with the default value. All this happens before the function is called; from within the function you just see that the value is taken and returned.
And there is yet another way to accomplish your task: Just use the lambda as if it would take a value: lambda i: i. If you call this, it complains about a missing argument.
But you can cope with that with the use of functools.partial:
ff = [functools.partial(lambda i: i, x) for x in range(100)]
ff[12]()
ff[54]()
This wrapper gets a callable and a number of arguments to be passed. The resulting object is a callable which calls the original callable with these arguments plus any arguments you give to it. It can be used here to keep locked to the value intended.