python - Generators and files -


when write:

lines = (line.strip() line in open('a_file'))

is file opened or file system accessed when start consume generator expression?

open() called upon construction of generator, irrespective of when or whether consume it.

the relevant spec pep-289:

early binding versus late binding

after discussion, decided first (outermost) for-expression should evaluated , remaining expressions evaluated when generator executed.

asked summarize reasoning binding first expression, guido offered [5]:

consider sum(x x in foo()). suppose there's bug in foo() raises exception, , bug in sum() raises exception before starts iterating on argument. exception expect see? i'd surprised if 1 in sum() raised rather 1 in foo(), since call foo() part of argument sum(), , expect arguments processed before function called.

otoh, in sum(bar(x) x in foo()), sum() , foo() bugfree, bar() raises exception, have no choice delay call bar() until sum() starts iterating -- that's part of contract of generators. (they nothing until next() method first called.)

see rest of section further discussion.


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