列表是否在 python 'for' 循环的每次迭代中实例化？

Question

I want to know if the list created (instantiated) and used in for loop will reduce efficiency of my program.

For example:

for i in range(1, 10000):
     print("This i = ", i)

Please tell me if the list [1,2,3,...,10000] (which is range(1,10000) ) will be generated (or instantiated) at every iteration or not. Because if Yes, then this is a huge overhead and inefficient program.

Actually I want to use it like this:

with open("bbc.txt", 'w', encoding='utf-8') as bbcFile:
    for headline in BS(REQ.get("https://www.bbc.com").text, 'html.parser').find_all('div', {'class':'media__content'}):
        bbcFile.write(" ".join(headline.text.split()) + "\n\n")

Answer 1

In a Python for-statement, as defined by the docs :

for_stmt ::=  "for" target_list "in" expression_list ":" suite
              ["else" ":" suite]

According to the aforementioned docs,

The expression list is evaluated once ; it should yield an iterable object. An iterator is created for the result of the expression_list . The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see Assignment statements), and then the suite is executed.

So no, whatever expression you are using in to produce an iterable is only evaluated once. You could test this out yourself:

>>> class MyIterable:
...     def __init__(self):
...         print("Initialized")
...     def __iter__(self):
...         yield from (1,2,3)
...
>>> for x in MyIterable():
...     print(x)
...
Initialized
1
2
3
>>>

Answer 2

For Python 3, no. range(1, 10000) creates a range object that produces items when necessary:

>>> range(1, 10000)
range(1, 10000)
>>> type(range(1, 10000))
<class 'range'>

So there is never a list [1, ..., 10000] stored in memory.

A great SO question to check out is this one , which explains the range object.