Python generator expression parentheses oddity

Question

I want to determine if a list contains a certain string, so I use a generator expression, like so:

g = (s for s in myList if s == myString)
any(g)

Of course I want to inline this, so I do:

any((s for s in myList if s == myString))

Then I think it would look nicer with single parens, so I try:

any(s for s in myList if s == myString)

not really expecting it work. Surprise! it does!

So is this legal Python or just something my implementation allows? If it's legal, what is the general rule here?

Sven Marnach · Accepted Answer · 2012-02-15 16:57:48Z

25

It is legal, and the general rule is that you do need parentheses around a generator expression. As a special exception, the parentheses from a function call also count (for functions with only a single parameter). (Documentation)

Note that testing if my_string appears in my_list is as easy as

my_string in my_list

No generator expression or call to any() needed!

answered Feb 15, 2012 at 16:57

Sven Marnach

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2 Comments

Ari Over a year ago

Thanks for the answer. my_string in my_list was the first thing I tried, but it failed to find a string that was present. I concluded that it was doing object comparison rather than value comparison, which is what I need. I'll check again.

Sven Marnach Over a year ago

@Ari: No, it does value comparison. I don't know what went wrong in your case.

Thomas Wouters · Accepted Answer · 2012-02-15 16:58:59Z

3

It's "legal", and expressly supported. The general rule is "((x)) is always the same as (x)" (even though (x) is not always the same as x of course,) and it's applied to generator expressions simply for convenience.

answered Feb 15, 2012 at 16:58

Thomas Wouters

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4 Comments

Sven Marnach Over a year ago

Just to clarify, x is not a place holder for just anything here. f((a, b)) is of course different from f(a, b).

Thomas Wouters Over a year ago

Yes, indeed, it's a placeholder for a single expression.

Ari Over a year ago

Can you provide a reference for this rule (that ((x)) is always the same as (x) )?

Sven Marnach Over a year ago

@Ari: See here: "A parenthesized expression list yields whatever that expression list yields." This usually doesn't apply to the parens in function calls, though -- it is a special exception for generator expressions.

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