为什么循环变量没有在python中更新

Question

This code is only printing 1 2 4 5..My question is that why p is not updated with the new array at 3rd iteration

p = [1, 2, [1, 2, 3], 4, 5]
for each in p:
  if type(each) == int:
    print each
  else:
    p = each

Actually to be precise when debugged the code I saw that it actually updating the value of p but each variable is not reinitialised again.

Answer 1

Because of if type(each) == int: line. Your third element is a list ( [1, 2, 3] ) and not an int, so it doesn't print anything.

Now what comes to changing the p variable: p is just a name for an object, not the object itself. If you do p = each inside the for loop, it doesn't affect the original object you're looping through, it just changes the name p to a local name, which points to a different object. As soon as that round of the loop ends, your for loop continues to do its business with the original object you were looping through.

So, notice that p = each doesn't change the existing object (the p you're looping through), it simply creates a new local name p which points to the value of each .

---

What you most likely want is something like this:

p = [1, 2, [1, 2, 3], 4, 5]
for each in p:
    if isinstance(each, list):
        for x in each:
            print x
    else:
        print each

This then again, this isn't recursive, and you'd need a function for that:

def print_elements(iterable):
    for element in iterable:
        if isinstance(element, list):
            print_elements(element)
        else:
            print element

---

If you want to unpack the values into one list to use them for something other than printing, you should use something like this:

def recursive_unpack(iterable):
    for element in iterable:
        if isinstance(element, list):
            yield from recursive_unpack(element)
        else:
            yield element

---

Why I'm using isinstance() instead of type() : Differences between isinstance() and type() in python

Also, if you want this to apply to all iterables (my last example) and not just lists: In Python, how do I determine if an object is iterable?

Answer 2

the issue is in the else statement, you're basically saying that if the each is not an int (in your case it's a list) then set the list p to the inside list.

I think that what you're trying to do can be accomplished by something like

p = [1, 2, [1, 2, 3], 4, 5]
for element in p:
if type(element) == int:
    print element
else:
    for otherElement in element:
        if type(otherElement) == int:
            print otherElement

the else statement in this case goes through the inside list and checks the elements it contains (otherElement)