How to set a Python variable to 'undefined'?

You probably want to set it to None.

variable = None

Check if variable is “defined”

is_defined = variable is not None

You could delete the variable, but it is not really pythonic.

variable = 1
del variable
try:
    print(variable)
except (NameError, AttributeError):
    # AttributeError if you are using "del obj.variable" and "print(obj.variable)"
    print('variable does not exist')

Having to catch a NameError is not very conventional, so setting the variable to None is typically preferred.

You can also define your var x as None

x = None

If you want to be able to test its ‘undefined state’, you should set it to None :

variable = None

and test with

if variable is None: 

If you want to clean stuff, you can delete it, del variable but that should be task of the garbage collector.

In light of the OP’s comments:

# check if the variable is undefined
try:
    x
# if it is undefined, initialize it
except NameError:
    x = 1

And like the rest said, you can delete a defined variable using the del keyword.

Here is a case when you actually want undef: function arguments that can have any value (including None), but we still need to know if the value was provided or not.

For example:

class Foo:
    """
    Some container class.
    """

    def pop(self, name, default):
        """
        Delete `name` from the container and return its value.

        :param name: A string containing the name associated with the
            value to delete and return.
        :param default: If `name` doesn't exist in the container, return
            `default`. If `default` is not given, a `KeyError` exception is
            raised.
        """
        try:
            return self._get_and_delete_value_for(name)
        except GetHasFailedError:
            if default is undefined:
                raise KeyError(name)
            else:
                return default

This is very much like dict.pop, and you need to know if default was given. One could fake that with *args, **kwargs, but that gets messy quickly, and having undef would really help.

IMO, the easiest approach for that is this:

_undef = object()


class Foo:
    """
    Some container class.
    """

    def pop(self, name, default=_undef):
        """
        Delete `name` from the container and return its value.

        :param name: A string containing the name associated with the
            value to delete and return.
        :param default: If `name` doesn't exist in the container, return
            `default`. If `default` is not given, a `KeyError` exception is
            raised.
        """
        try:
            return self._get_and_delete_value_for(name)
        except GetHasFailedError:
            if default is _undef:
                raise KeyError(name)
            else:
                return default

The leading underscore implies that the variable is private to the module and shouldn’t be used outside of it, which suggests that _undef should not be used as an argument value, making it a good detector for "this value is undefined".

Use this:

undefined = "undefined"
# del x isn't effective, so use "x = undefined"
x = undefined