Detailed explanation of how the for loop works in Python

If you are not very clear about the for loop in Python, then I suggest you read this article. This article mainly introduces you to the relevant information about how the for loop in Python works. It is very detailed and has certain reference and learning value for everyone. Friends who need it can take a look below.

Preface

for...in is the most commonly used statement by Python programmers. The for loop is used to iterate the containerObject# Elements in ##, these objects can be lists, tuples, dictionaries, collections, files, or even custom classes or functions, for example:

acts on lists

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

Applies to tuples

>>> for i in ("zhang", "san", 30):
...  print(i)
...
zhang
san
30

Applies to string

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

Action on collection

>>> for i in {"a","b","c"}:
...  print(i)
...
b
a
c

Action In dictionary

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

Apply to file

##

>>> for line in open("requirement.txt"):
...  print(line, end="")
...
Fabric==1.12.0
Markdown==2.6.7

Some people may not know You have to ask, why do so many different types of objects support the for statement? What other types of objects can be used in the for statement? Before answering this question, we must first understand the execution principle behind the for loop.

The for loop is the process of iterating the 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? Can any object be used? Try customizing a class first and see if it works:

>>> 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: &#39;MyRange&#39; object is not iterable

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

Iterable objects need to implement the iter method and return an iterator. What is an iterator? Iterators only need to implement the next method. Now let's verify why the list supports iteration:

>>> x = [1,2,3]
>>> its = x.iter() # x有此方法，说明列表是可迭代对象
>>> its
<list_iterator object at 0x100f32198>

>>> its.next() # its有此方法，说明its是迭代器
1
>>> its.next()
2
>>> its.next()
3
>>> its.next()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration

From the test results, the list is an iterable object because it implements the iter method and returns An iterator object (list_iterator) because it implements the next method. We see that it continuously calls the next method, which actually continuously iterates to obtain the elements in the container until there are no more elements in the container and a StopIteration exception is thrown.

So how does the for statement loop? At this point, I am afraid you have guessed it. The steps are:

# First determine whether the object is an iterable object. If not, an error will be reported directly and a TypeError exception will be thrown. If so, , call the iter method and return an iterator
Continuously call the next method of the iterator, each time returning a value in the iterator in order
At the end of the iteration, if there are no more elements,
• throw an exception

  StopIteration. Python will handle this exception by itself and will not expose it to developers
  

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

The previous MyRange error is because it does not implement these two methods in the iterator protocol. Now continue 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:
   # 达到某个条件时必须抛出此异常，否则会无止境地迭代下去
   raise StopIteration()

Because it implements next method, so MyRange itself is already an iterator, so iter returns the object itself. Now try using it in a for loop:

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

Have you noticed that the custom MyRange function is very similar to the built-in function range. The essence of a for loop is to continuously call the next method of the iterator until a StopIteration exception occurs, so any iterable object can be used in a for loop.

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