What is this 1 between the colons in the SyntaxWarning?

Python Help
1 
>>> a = 256
>>> b = 256
>>> 256 is a
<python-input-12>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True
>>> 256 is b
<python-input-13>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True
>>> a is b
True
>>> 256 is 256
<python-input-15>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True
>>> a is 256
<python-input-16>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True
>>> a is b
True
>>> (a is 256) and (b is 256) and (256 is a) and (256 is b) and (a is b)
<python-input-18>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
<python-input-18>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
<python-input-18>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
<python-input-18>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True
>>>
2

It’s the line number, which in the repl case is relative to each entry:

>>> if True
...     256 is a
...     256 is b
<python-input-4>:2: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
<python-input-4>:3: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True
True