Question

In the following examples,

[f(a) for a in range(10) if f(a) < 50]
{k: f(a) for k in range(10) if f(a) < 50}

Is it possible to eliminate the repeated f(a), without introducing a wrapping list/dictionary comprehension?

Answer 1

You can avoid the repeated call to f(a) by introducing a dummy iteration over a single-element list:

(b for a in range(10) for b in [f(a)] if b < 50)

or by introducing a sub-generator:

(b for b in (f(a) for a in range(10)) if b < 50)

They are a bit inelegant, but they work, and are reasonably clear.

You’ll probably be interested in this long thread on the python-ideas mailing list, in which Mathias Panzenböck proposes the addition of new syntax to the language to handle this case more elegantly. Generally, the response from the Python developers was that the problem isn’t severe enough to warrant the additional complexity.