Recently in an implementation of a functionality, following error keeps showing and drives me crazy.
Traceback (most recent call last):
File "test_additional_parameter.py", line 13, in <module>
fn(3)
TypeError: <lambda>() takes 1 positional argument but 2 were given
It is a really weird problem. It is a function which has only one parameter.
Minimal case
from pprint import pprint
class Test:
fn = lambda x: pprint(x)
a = Test()
fn = a.fn
fn(3)
It is obvious that you may guess fn is a function of an instance and it will receive
an argument self from the python interpreter.
methods are functions, they are simply attached to the class, and when that function is called from an instance it gets that instance passed implicitly as the first argument automagically
After changing the method reference using a class, it works as expected.
from pprint import pprint
class Test:
fn = lambda x: pprint(x)
@classmethod
def fn1(cls, x):
pprint(x)
a = Test.fn
a(3)
b = Test.fn1
b(4)
Methods v.s. Functions, Arguments v.s. Parameters
We have already known the difference of a method and a function. There is also a similar confusing pair: Parameters and Arguments. Parameters are the signature of a method or a function while arguments are passed to the corresponding parameters’ posistion.