What does this Python syntax do?

Question

New to python and trying to understand what does following syntax doing ?

  def testMissingConfig(self):
        """ if config is missing, the config is valid """

        input_args = self.buildmock()

        validation_errors = [
            x
            for x in self.validator.validate(
                ValidatorArguments(input_args=input_args)
            )
            if x
        ]
        validation_keys = {x.key for x in validation_errors}
        self.assertEmpty(validation_keys)

Especially the array initialization for "validation_errors"

This should have been closed as a duplicate of [What does “list comprehension” mean? How does it work and how can I use it?](https://stackoverflow.com/q/34835951/2745495) — Gino Mempin, Mar 31 '20 at 01:49

score 0 · Accepted Answer · answered Mar 17 '20 at 03:34

It is called List comprehension. Here you can combine assignments, loops, functions all in one block.

One of the big advantages of list comprehension is that they allow developers to write less code that is often easier to understand.

Syntax:

[expression for item in list]

Example:

number_list = [ x for x in range(20) if x % 2 == 0]
print(number_list)

Here the numberlist loops over 0 to 20 and gives even numbers 0,2,4...20 as result.

Similarly in your code, validation_errors will store x if x exists(not null)

Lambda functions can also be used to create and modify lists in less lines of code.

Reference: https://www.programiz.com/python-programming/list-comprehension

What does this Python syntax do?

1 Answers1