问题
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.
回答1:
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.
回答2:
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.
来源：https://stackoverflow.com/questions/45849120/python-tuple-vs-generator