Python transform and filter list with for / if

Question

Is there a way to both transform and filter in a single list comprehension, i.e.:

def transform(el):
    if some_condition(el):
        return None

    return complex_logic(el)

def main():
    transformed = [transform(el) for el in some_list if transform(el) != None]

but avoid calling transform twice? I.e. assign it to a variable, something like (in pseudo-Python):

def main():
  transformed = [transformed for el in some_list let transformed = transform(el) if transformed != None]

Answer 1

Since Python 3.8 you can use walrus operator :=:

def main():
    return [res for el in some_list if (res := transform(el)) is not None]

This way the result of calling to the transform function is stored in res then you can use it in the expression part of your list comprehension.