Python built-in function (--iter)

English documents:

iter(object[, Sentinel])

Return aniteratorObject. The first argument is interpreted very differently depending on the presence of the second argument. Without a second argument,ObjectMust be a collection object which supports the iteration protocol (the__iter__()method), or it must support the sequence protocol (the__getitem__()Method with an integer arguments starting at0). If it does not support either of those protocols,TypeErroris raised. If the second argument,Sentinel, is given, thenObjectMust be a callable object. The iterator created in this case would callObjectWith no arguments for each call to its__next__()Method If the value returned is equal toSentinel,StopIterationWould be raised, otherwise the value would be returned.

One useful application of the second form of are to iter() read lines of a file until a certain line is reached. The following example reads a file until the readline() method returns an empty string:

With open ('mydata.txt') as FP:    for in'):        Process_line (line)

Description

1. Function function returns an object that can be iterated.

2. When the second parameter is not provided, the first argument must be a collection (dictionary, collection, immutable collection) that supports an iterative protocol (that is, the __iter__ () method is implemented), or a sequence protocol (that is, the __getitem__ () method is implemented, and the method receives an integer parameter starting from 0) Sequence (tuple, list, string), otherwise an error will be expected.

>>> A = iter ({'A': 1,'B': 2})#Dictionary Collection>>>a<dict_keyiterator Object at 0x03fb8a50>>>>Next (a)'A'>>>Next (a)'B'>>>Next (a) Traceback (most recent): File"<pyshell#36>", Line 1,inch<module>Next (a) Stopiteration>>> A = ITER ('ABCD')#string Sequence>>>a<str_iterator Object at 0x03fb4fb0>>>>Next (a)'a'>>>Next (a)'b'>>>Next (a)'C'>>>Next (a)'D'>>>Next (a) Traceback (most recent): File"<pyshell#29>", Line 1,inch<module>Next (a) Stopiteration

3. When Sentinel provides the second parameter, the first argument must be a callable object. Creates an iterative object that invokes the callable object when the __next__ method is called, and throws when the return value and the Sentinel value are equalStopIteration异常， 终止迭代。

#Defining Classes>>>classitertest:def __init__(self): Self.start=0 Self.end= 10defGet_next_value (self): current=Self.startifCurrent <Self.end:self.start+ = 1Else:            Raisestopiterationreturn Current>>> itertest = Itertest ()#Instantiating Classes>>> a = iter (itertest.get_next_value,4)#Itertest.get_next_value is a callable object with a Sentinel value of 4>>>a<callable_iterator Object at 0x03078d30>>>>Next (a) 0>>>Next (a)1>>>Next (a)2>>>Next (a)3>>> Next (a)#iteration to 4 terminationTraceback (most recent): File"<pyshell#22>", Line 1,inch<module>Next (a) Stopiteration

Python built-in function (--iter)