Python3 cant detect<class 'NoneType'>-CodePudding

I am running some python3 code which will occasionally get a list, dict, and None.

        fieldType = type(raw_data[root_key].get("oslc_cm:ChangeRequest"))
        print('fieldType=')
        print(fieldType)
        if fieldType is None:
            print('its none')
        else:
            print('its not none')

this works for everything except when fieldType is 'None':

fieldType=
<class 'collections.OrderedDict'>
its not none
#this output works as expected

but when fieldType is <class 'NoneType'> it reports it as being 'not none'

fieldType=
<class 'NoneType'>
its not none

why cant my code correctly identify when an object is of type 'None'?

CodePudding user response：

fieldType is <class 'NoneType'>, which is different from None. It can never be None, because type always returns some type.

Looks like you want

raw_data[root_key].get("oslc_cm:ChangeRequest") is None

instead of

fieldType is None

CodePudding user response：

None != type(None)

type(None) is a <class 'type'> object. The correct way to chech a type of a variable in python is to use isinstance(). Then your code would look like this:

NoneType = type(None)
fieldType = type(raw_data[root_key].get("oslc_cm:ChangeRequest"))
print('fieldType=')
print(fieldType)
if isinstance(fieldType, NoneType):
    print('its none')
else:
    print('its not none')

CodePudding user response：

This is a common mistake. The proper way to test for a value being None is to use is or is not, not using equality tests:

if fieldType is None:
    print('its none')
else:
    print('its not none')

the get() method returns None (and hence evaluates to False) when there are no items in the dictionary. This means you'd get the same result by using if not raw_data[root_key].get("oslc_cm:ChangeRequest"): which means "if there's no entry for oslc_cm:ChangeRequest in raw_data[root_key]". Because of this, you can write your code like this instead:

    fieldType = type(raw_data[root_key].get("oslc_cm:ChangeRequest"))
    if fieldType is not None:
        print('its not none')
    else:
        print('its none')