Isinstance（）不起作用

Question

For some reason I can't get isinstance() to work on Python 2.7.2

def print_lol(the_list, indent=False, level=0):
    for item in the_list:
        if isinstance(item, list):
            print_lol(item, indent, level+1)
        else:
            print(item)

And when I compile and run it:

>>> list = ["q", "w", ["D", ["E", "I"]]]
>>> print_lol(list)

I get the error message:

if isinstance(item, list):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

What am I mising?

Answer 1

You've named your variable list :

>>> list = ["q", "w", ["D", ["E", "I"]]]

Which is hiding the built-in list .

Once you've renamed your variable, restart Python (or your IDE). Since your list is a global variable, it will stick around otherwise.

Answer 2

This is where you have issue

>>> list = ["q", "w", ["D", ["E", "I"]]]

overwriting python named types like list binds the variable name with a new object instance. So later when you tried isinstance with list it failed.

When ever you create new variable, please refrain from naming it differently from built-in and not to conflict with the namespace.

In this example using the following would work like a breeze

>>> mylist = [w for w in mylist if len(w) >= 3 and diff(w)]
>>> isinstance(mylist,list)
True

Please note, if you have polluted the namespace and you are running in IDLE, restarting IDLE is a good alternative.

Answer 3

The problem is you have shadowed the built-in list by assigning to a variable called list :

list = ["q", "w", ["D", ["E", "I"]]]

You should try to avoid using any of the built-in names for variables, because this will often result in confusing errors. The solution is to use a different name for your variable.

Answer 4

您将要覆盖list ，尝试print list前isinstance诊断这类问题。