Python file variable - what is it? -

i started python, , since background in more low-level languages (java, c++), cant things.

so, in python 1 can create file variable, opening text file, example, , iterate through lines this:

f = open(sys.argv[1]) line in f:     #do 

however, if try f[0] interpreter gives error. structure f object have , how know in general, if can apply for ... in ... : loop object?

f file object. documentation lists structure, i'll explain indexing/iterating behavior.

an object indexable if implements __getitem__, can check calling hasattr(f, '__getitem__') or calling f[0] , seeing if throws error. in fact, that's error message tells you:

typeerror: 'file' object has no attribute '__getitem__' 

file objects not indexable. can call f.readlines() , return list of lines, indexable.

objects implement __iter__ iterable for ... in ... syntax. there 2 types of iterable objects: container objects , iterator objects. iterator objects implement 2 methods: __iter__ , __next__. container objects implement __iter__ , return iterator object, you're iterating over. file objects own iterators, implement both methods.

if want next item in iterable, can use next() function:

first_line = next(f) second_line = next(f) next_line_that_starts_with_0 = next(line line in f if line.startswith('0')) 

one word of caution: iterables aren't "rewindable", once progress through iterable, can't go back. "rewind" file object, can use f.seek(0), set current position beginning of file.