Python follow up on yield as a function argument

Asked Jul 12 '17 at 10:22

Active Jul 12 '17 at 10:22

Viewed 73 times

I was reading this question

How does this lambda/yield/generator comprehension work?

I would like to understand the language constructs in a bit more detail. Why are the brackets necessary?

(yield 55)

as opposed to

yield 55

Why does it only work in a generator expression when it is being passed as an argument to a function? If I do the following

 a = lambda x: 1
 a((yield 55))
 SyntaxError: 'yield' outside function
 # The error here implies that it must be used within a function... not a generator expression

When running the following:

a = lambda x: 1
list(a((yield 55)) for b in range(5))
[55,
 1,
 55,
 1,
 55,
 1,
 55,
 1,
 55,
 1]

Why does this yield produce a value? Who is calling next on it? The order of the evaluation is not clear by the syntax.

asked Jul 12 '17 at 10:22

Har

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1

Wait what, you can do that? – cs95 Jul 12 '17 at 10:27
yes, that was my first impression too – Har Jul 12 '17 at 10:28
2

For an extra surprise, add some square brackets around the generator expression (i.e. directly inside the parentheses for the `list()` call. – Sven Marnach Jul 12 '17 at 10:32
1

It doesn't have to be passed as an argument to a function. `list((yield 55) or b for b in range(5))` gives the same result. I don't know how the parenthesis around `yield 55` works, though. – Håken Lid Jul 12 '17 at 10:50
2

should probably be considered as a bug. – Jul 12 '17 at 10:59
1

@HåkenLid Who needs `zip()` when we can use this construct? :p – Chris_Rands Jul 12 '17 at 11:40

Python follow up on yield as a function argument

0 Answers0