The working principle of for loop in Python is detailed

For example:

Action on list

>>> for elem in [1,2,3]:
...     print(elem)
...
1
2
3

Action on String

>>> for c in "abc":
...     print(c)
...
a
b
c

Act on a dictionary

>>> for k in {"age":10, "name":"wang"}:
...     print(k)
...
age
name

Some people may not ask why so many different types of objects are supported for Statement, what other types of objects can be used in the for What about the statement? Before answering this question, we must first understand for The execution principle behind the loop.

for Loop is the process of iterating over a container. What is iteration? Iteration is to read elements from a container object one by one until there are no more elements in the container. So, which objects support iterative operations? Is any object ok? Try customizing 1 class first to see if it is OK:

>>> class MyRange:
...     def __init__(self, num):
...         self.num = num
...
>>> for i in MyRange(10):
...     print(i)
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'MyRange' object is not iterable

The error stack log tells us very clearly that MyRange is not an iterable object, so it cannot be used for iteration, so what kind of object is an iterable object ( iterable )?

Iterable objects need to implement __iter__ Method and returns 1 iterator. What is an iterator? The iterator only needs to implement the __next__ Method. Now let's verify why the following list supports iteration:

>>> x = [1,2,3]
>>> its = x.__iter__() # x This means that the list is an iterable object 
>>> its
<list_iterator object at 0x100f32198>

>>> its.__next__()  # its With this method, explain its Is an iterator 
1
>>> its.__next__()
2
>>> its.__next__()
3
>>> its.__next__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

From the experimental results, the list is an iterable object because it implements the __iter__ Method and returns 1 iterator object ( list_iterator ), because it implements the __next__ Method. We see it constantly calling __next__ Method, in fact, iterates to get the elements in the container until no more elements in the container are thrown StopIteration Until it's abnormal.

Then for How do statements loop? Here, I'm afraid you guessed, its steps are:

First judge whether the object is an iterative object, if not, report an error directly and throw it TypeError Exception, if yes, call __iter__ Method that returns 1 iterator Constantly calling the iterator's __next__ Method, returning 1 value in the iterator in sequence at a time At the end of the iteration, when there are no more elements, an exception is thrown StopIteration This exception python It will be handled by itself and will not be exposed to developers

The same is true for tuples, dictionaries, and strings. After understanding the execution principle of for, we can implement our own iterators to use in for loops.

Front MyRange The error is reported because it does not implement these two methods in the iterator protocol, and now it continues to improve:

class MyRange:
    def __init__(self, num):
        self.i = 0
        self.num = num

    def __iter__(self):
        return self

    def __next__(self):
        if self.i < self.num:
            i = self.i
            self.i += 1
            return i
        else:
            #  This exception must be thrown when a certain condition is met, otherwise it will iterate indefinitely 
            raise StopIteration()

Because it implements __next__ Method, so MyRange It is already an iterator, so __iter__ What is returned is the object itself self . Now try it in the for loop:

for i in MyRange(3):
    print(i)
#  Output 
 0
 1
 2

Have you found that custom MyRange Functions and built-in functions range Very similar. for The essence of a loop is to constantly call the iterator __next__ Method until there is a StopIteration Exception, so any iterable object can be used in the for In the loop.