iter(object, sentinel)

Returns test

object	??
sentinel	??

REMARKS

* No prefix required - Core
* Return an iterator object. The first argument is interpreted very differently depending on the presence of the second argument. Without a second argument, object must be a collection object which supports the iterable protocol (the __iter__() method), or it must support the sequence protocol (the __getitem__() method with integer arguments starting at 0). If it does not support either of those protocols, TypeError is raised. If the second argument, sentinel, is given, then object must be a callable object. The iterator created in this case will call object with no arguments for each call to its __next__() method; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned.
* One useful application of the second form of iter() is to build a block-reader. For example, reading fixed-width blocks from a binary database file until the end of file is reached:
* See also Iterator Types.

from functools import partial 
with open('mydata.db', 'rb') as f: 
    for block in iter(partial(f.read, 64), b''):  
        process_block(block)