[kazinator]

Are you saying that a non-mutating variant of the above works? For instance:

   foo.on_click(lambda:x func(x, whatever))

[dragonwriter]

yeah, except that the syntax is

  lambda x: func(x, whatever)

In fact, if x is an object that stores a mutable numeric cell, you could even do a mutating:

  lambda x: x.add(1)

Mutation isn't prohibited in Python lambdas, statements (including assignments, which are statements rather than expressions in Python), however, are.