One of the most common problems for new Kivy users is misunderstanding how the bind method works, especially amongst newer Python users who haven’t fully formed their intuition about function calls. For instance, a user will write code like:
some_screenmanager.bind(current=a_function(arg1, arg2))
Here, the idea is that when the current property changes, it
will call a_function with the arguments arg1 and
arg2.
The problem is that Python itself doesn’t work like
this; the bind method doesn’t know about the existence of
a_function or its arguments, it only receives the result of
this function call. This often leads to confusion when a user doesn’t
understand why the binding is only called once, during the declaration
of the binding.
Stepping back, our real goal is to call a_function with the
given arguments, but bind needs to be passed a function if it
is to work correctly. That means we can solve our problem by creating
a new function with these arguments already passed (and discarding the
extra arguments automatically passed by bind).
It’s usually convenient to do this with the partial function
from the functools module:
from functools import partial
some_screenmanager.bind(current=partial(a_function,arg1, arg2))
partial returns a new function that will automatically be
passed arg1 and arg2, exactly as we want. You can also pass kwargs
this way.
This isn’t the only way to solve the problem, we could have created a
lambda function (though the syntax is longer and can have scope
problems) or an entire new function with def syntax, but both
of these are more complicated than the simple use of
partial. So if you need to do a binding in Python, look at
this way first!