Python nested list of integers not returning as expected

Question

I'm writing this function that checks if a list of lists would be a valid sudoku puzzle. When I'm checking the lists for valid integers I'm getting unexpected results.

For example:

lst = [[1,2,3],[2,3,1],[4,2,1]]
for i in lst: 
  for v in i:
    print type(v)

<type 'int'>   #all the way through

for i in lst:
  for v in i:
    if v is int:
      print True

Prints nothing, and when I through in:

for i in lst:
  for v in i:
    if v is not int:
      print False

Prints all False? Not sure about what is going on, especially with the type showing they're integers.

Answer 1

Instead of saying

if v is int:

Which is asking if v is the actual type int

Say

if isinstance(v, int):

which says v is an instantiated int (or subclass)

Here is an example, first with an integer (or instantiated int )

>>> v = 17
>>> type(v)
<type 'int'>
>>> v is int
False
>>> isinstance(v, int)
True
>>> 

Next with a type

>>> v = int
>>> type(v)
<type 'type'>
>>> v is int
True
>>> isinstance(v, int)
False
>>> 

Answer 2

No integer is the same object as int . You want to use isintance .

You don't want to use type(v) == int , because that will evaluate to False if v is a subclass of int , which is in most cases not the desired behaviour at all; if you do really, really want that, do type(v) is int , because is is the expected style for comparison with a specific object (which is what a type is).

Answer 3

v is not int

is the same as saying, but a more readable version of

not v is int

is compares the references of each operand (not the types) to see if they are the same. Here is an example of that:

>>> x = (1,2,3)
>>> y = (1,2,3)
>>> x == y
True
>>> x is y
False

So v is int obviously evaluates to False . not turns False into True and the if statement prints.

Use isinstance(v, int) for the correct result.

Answer 4

回答您应该使用isinstance的答案是正确的，但是另一种替代方法是使用if type(v) == int 。