Are Python3.5 tuple comprehension really this limited?

Question

I\'ve been loving the tuple comprehensions added to Python3.5:

In [128]: *(x for x in range(5)),
Out[128]: (0, 1, 2, 3, 4)

However, when I

Answer 1

TLDR: If you want a tuple, pass a generator expression to tuple:

{idx: tuple(x for x in range(5)) for idx in range(5)}

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There are no "tuple comprehensions" in Python. This:

x for x in range(5)

is a generator expression. Adding parentheses around it is merely used to separate it from other elements. This is the same as in (a + b) * c, which does not involve a tuple either.

The * symbol is for iterator packing/unpacking. A generator expression happens to be an iterable, so it can be unpacked. However, there must be something to unpack the iterable into. For example, one can also unpack a list into the elements of an assignment:

*[1, 2]                         # illegal - nothing to unpack into
a, b, c, d = *[1, 2], 3, 4      # legal - unpack into assignment tuple

Now, doing *, combines * unpacking with a , tuple literal. This is not useable in all situations, though - separating elements may take precedence over creating a tuple. For example, the last , in [*(1, 2), 3] separates, whereas in [(*(1, 2), 3)] it creates a tuple.

In a dictionary the , is ambiguous since it is used to separate elements. Compare {1: 1, 2: 2} and note that {1: 2,3} is illegal. For a return statement, it might be possible in the future.

If you want a tuple, you should use () whenever there might be ambiguity - even if Python can handle it, it is difficult to parse for humans otherwise.

When your source is a large statement such as a generator expression, I suggest to convert to a tuple explicitly. Compare the following two valid versions of your code for readability:

{idx: tuple(x for x in range(5)) for idx in range(5)}
{idx: (*(x for x in range(5)),) for idx in range(5)}

Note that list and dict comprehensions also work similar - they are practically like passing a generator expression to list, set or dict. They mostly serve to avoid looking up list, set or dict in the global namespace.

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I feel like this is a bit more of a problem since comprehsions can be important for performance in some situations.

Under the covers, both generator expressions and list/dict/set comprehensions create a short-lived function. You should not rely on comprehensions for performance optimisation unless you have profiled and tested them. By default, use whatever is most readable for your use case.

dis.dis("""[a for a in (1, 2, 3)]""")
  1           0 LOAD_CONST               0 ( at 0x10f730ed0, file "", line 1>)
              2 LOAD_CONST               1 ('')
              4 MAKE_FUNCTION            0
              6 LOAD_CONST               5 ((1, 2, 3))
              8 GET_ITER
             10 CALL_FUNCTION            1
             12 RETURN_VALUE