Python 海龟/tkinter 事件中的部分 vs lambda

Question

In Python turtle , if I want to pass an event handler arguments that differ from what the event system specifies, I can use a lambda to bridge the difference:

from turtle import Screen, Turtle
from functools import partial

def change_color(color, x=None, y=None):
    screen.bgcolor(color)

screen = Screen()

screen.onclick(lambda x, y: change_color('blue'))

screen.mainloop()

Or I can use the partial function imported from functools to replace the lambda with:

screen.onclick(partial(change_color, 'blue'))

And that works fine . Returning to our original program, we can replace our onclick() event with an ontimer() event, updating our lambda , and everything works fine :

screen.ontimer(lambda: change_color('blue'), 1000)

But, when we replace this lambda with a partial :

screen.ontimer(partial(change_color, 'blue'), 1000)

It fails immediately (not when the timer would have fired) with:

Traceback (most recent call last):
  File "test.py", line 9, in <module>
    screen.ontimer(partial(change_color, 'blue'), 1000)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/turtle.py", line 1459, in ontimer
    self._ontimer(fun, t)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/turtle.py", line 718, in _ontimer
    self.cv.after(t, fun)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/tkinter/__init__.py", line 755, in after
    callit.__name__ = func.__name__
AttributeError: 'functools.partial' object has no attribute '__name__'
>

Since turtle sits atop tkinter , and tkinter is implicated in the stack trace, we can go down a level:

import tkinter as tk
from functools import partial

def change_color(color):
    root.configure(bg=color)

root = tk.Tk()

root.after(1000, change_color, 'blue')

root.mainloop()

Which works fine . We can also do:

root.after(1000, lambda: change_color('blue'))

Which works fine . But when we do:

root.after(1000, partial(change_color, 'blue'))

it again immediately fails with:

Traceback (most recent call last):
  File "test.py", line 9, in <module>
    root.after(1000, partial(change_color, 'blue'))
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/tkinter/__init__.py", line 755, in after
    callit.__name__ = func.__name__
AttributeError: 'functools.partial' object has no attribute '__name__'
>

The functools documentation for partial states its return value will behave like a function but clearly it's different, if not lacking, somehow. Why is that? And why does tkinter/turtle accept partial functions as click event handers, but not as timer event handlers?

Answer 1

Why is that?

It's designed this way. By default, partial stores the packed function without attributes, however they are still available:

partial_func = partial(change_color, 'blue')
print(partial_func.func.__name__)

You should use update_wrapper function (or a decorator counterpart ) to explicitly set correct options:

from turtle import Screen, Turtle
from functools import partial, update_wrapper

def change_color(color, x=None, y=None):
    screen.bgcolor(color)

def partial_change_color(color):
    partial_f = partial(change_color, color)
    update_wrapper(partial_f, change_color)

    return partial_f

screen = Screen()

screen.ontimer(partial_change_color('blue'), 1000)

screen.mainloop()

And why does tkinter/turtle accept partial functions as click event handers, but not as timer event handlers?

Again, it's designed this way. Because binding and scheduling algorithms are slightly different in the tkinter wrapper, which can be observed if you track down your error .

tkinter creates additional wrapper callit , which handles unscheduling of a target function using __name__ (hence, AttributeError ), while the binding does not have such an algorithm for implicit unbinding .

Answer 2

From here , it looks like functools.partial does not copy the __module__ and __name__ attributes from the inner function. You can work around it by defining __name__ manually:

import tkinter as tk
from functools import partial

def change_color(color):
    root.configure(bg=color)

root = tk.Tk()

c = partial(change_color, 'blue')
c.__name__ = "c"

root.after(1000, c)

root.mainloop()