用括号而不是方括号索引语法是什么意思？

Question

In Python (matplotlib package), I created an object of ListedColormap using the following line:

listed_color_map = ListedColormap(('red', 'green', 'blue'))

Then I saw a code retrieving the colors (eg green) like this:

listed_color_map(1)

Calling the previous line returns:

(0.0, 0.5019607843137255, 0.0, 1.0)

I have never seen this kind of Indexing with parenthesis (square brackets don't work here). This looks for me exactly as like as calling a method. What is this syntax of retrieving the values from the ListedColorMap exactly?

And it can be also called like this:

listed_color_map = ListedColormap(('red', 'green', 'blue'))(1)

This seems to me just like calling a method immediately after calling the constructor which is a bit weird for me (in C family there is no equivalent to this).

Answer 1

It means that the instances are "callable" ie the class (or a parent of it) implements the __call__ method. This dunder method allows to "call" the instances with the parantheses.

For your very example, it turns out the parent of ListedColormap class, which is ColorMap class, implements dunder call method. We can see the source code of ListedColormap and its parent that implements that interface.

The doc of the __call__ function reads

Returns
-------
Tuple of RGBA values if X is scalar, otherwise an array of
RGBA values with a shape of ``X.shape + (4, )

where X is your input, 1. In your second example, you are more or less doing the same thing: you're generating an instance of ListedColormap class and immediately "call"ing it with a scalar argument.