The internal mechanism of Python-for loop

In Python, the for loop can be used to iterate the elements in the container object, where the container object includes a list (list), a tuple (tuple), a dictionary (dict), a set (set), etc. But why can these objects be manipulated using a for loop?

First, define a simple class to try:

classTestRange:
 def __init__(self, num):
 self.num = num
for i inTestRange(10):print(i)

# Output
Traceback(most recent call last):
 File "<stdin ", line 1,in<module 
TypeError:'TestRange' object is not iterable

The error message indicates that the'TestRange' object is not an iterable object. So, what is an iterable object?

In an iterable object, a magic method of __iter__ needs to be implemented, and the return value of this method needs to be an iterator. So, what is an iterator?

Iterators only need to implement the __next__ magic method.

Take list as an example:

 nums =[13,12,33]
 iter_ret = nums.__iter__() #x has this method, indicating that the list is iterable, and the method returns an iterator
 iter_ret
< list_iterator object at 0x100f32198 

 iter_ret.__next__()13
 iter_ret.__next__()12
 iter_ret.__next__()33
 iter_ret.__next__()Traceback(most recent call last):
 File "<stdin ", line 1,in<module 
StopIteration

analysis:

As shown above, the __iter__ method is implemented in the list nums, and an iterator is returned, and the __next__ method is implemented in the iterator. In the process of calling __next__, it is returning the elements in nums until the error of StopIteration occurs.

In fact, the function of the for statement is similar. The internal mechanism of the for statement is:

First judge whether the object is an iterable object, that is, whether there is a __iter__ method, if it exists, call the __iter__ method to return an iterator; otherwise, directly throw a TypeError exception;
Call the iterator's __next__ method continuously, and each call iteratively obtains the current value in sequence;
After all the elements are iterated, the exception StopIteration will be thrown, and the python interpreter will handle this exception;

The previous TestRange reported an error because it did not implement the two methods in the iterator protocol. Now continue to improve:

classTestRange:
 def __init__(self, _max):
 self.i =0
 self._max = _max

 def __iter__(self):return self

 def __next__(self):if self.i < self._max:
  i = self.i
  self.i +=1return i
 else:
  # When the stop condition is reached, this exception is thrown
  raise StopIteration()

# carry out testing
for i inTestRange(3):print(i)
# Output
 012

analysis:

Because this class has already implemented the __next__ method, the object created based on this class is itself an iterator. And because the iterable object needs to have the __iter__ method and return an iterator, so __iter__ returns the object itself self.

to sum up

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