Python tuple vs generator

Question

I am having a problem understanding why one of the following line returns generator and another tuple.

How exactly and why a generator is created in the second line, while in the third one a tuple is produced?

sample_list = [1, 2, 3, 4]
generator = (i for i in sample_list)
tuple_ = (1, 2, 3, 4)

print type(generator)
<type 'generator'>

print type(tuple_)
<type 'tuple'>

Is it because tuple is immutable object and when I try to unpack list inside (), it can't create the tuple as it has to change the tuple tuple.

Paul92 · Accepted Answer · 2017-08-23 21:02:35Z

28

You can imagine tuples as being created when you hardcode the values, while generators are created where you provide a way to create the objects.

This works since there is no way (1,2,3,4) could be a generator. There is nothing to generate there, you just specified all the elements, not a rule to obtain them.

In order for your generator to be a tuple, the expression (i for i in sample_list) would have to be a tuple comprehension. There is no way to have tuple comprehensions, since comprehensions require a mutable data type.

Thus, the syntax for what should have been a tuple comprehension has been reused for generators.

answered Aug 23, 2017 at 21:02

Paul92

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2 Comments

visitor Over a year ago

Actually there is tuple comprehension: xs = [1, 2, 3]; ys = tuple(x for x in xs); type(ys)

Ian Goldby Over a year ago

@visitor That's not a tuple comprehension. x for x in xs is a generator, and this is then converted to a tuple.

Ry- · Accepted Answer · 2017-08-23 21:02:26Z

7

Parentheses are used for three different things: grouping, tuple literals, and function calls. Compare (1 + 2) (an integer) and (1, 2) (a tuple). In the generator assignment, the parentheses are for grouping; in the tuple assignment, the parentheses are a tuple literal. Parentheses represent a tuple literal when they contain a comma and are not used for a function call.

answered Aug 23, 2017 at 21:02

Ry-♦

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2 Comments

Gaurang Shah Over a year ago

I under (1+2) is int and (1, 2) would be function call, what i don't understand is what you mean by in the generator assignment.

Ry- Over a year ago

@GaurangShah: When you’re assigning generator = …. … for … in … is a generator when it’s inside parentheses.

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