Unsupported operand type(s) for +: 'NoneType' and 'str'

Python Help
1

In the book << professional python >> (author: Luke Sneeringer) page3-page4 (or say page33-page34)

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def decorated_by(func):
    func.__doc__ +=  '\nDecorated by decorated_by.'
    return func

def add(x,y):
    return x+y

add = decorated_by(add)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in decorated_by
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'

My python version:

python3 --version
Python 3.11.2

How to fix it?

2

Your add function is different from the one in the book, which is why the decorator function is not working for you. Can you spot the difference?

3

I can’t spot the difference,please tell me why in detail.

4

The books version is 3 lines long, your version is 2 lines long.

5

The difference is this line:

"""Return the sum of x and y."""

which is present in the example but missing from your function. It is a docstring. A common misconception is that docstrings are comments; this is not the case. Docstrings are part of your program and (usually) exist at runtime.

The docstring of an object is stored in the object’s __doc__ attribute. If the object has no docstring, this attribute is None.

The decorated_by function tries to add a string to a function’s existing docstring. If the input function does not have a docstring, as in your case, this fails because a str cannot be added to None.

1 Like
6

Try (untested)

def decorated_by(func):
    if func.__doc__ is not None:
        func.__doc__ +=  '\nDecorated by decorated_by.'
    else:
        func.__doc__ =  'Decorated by decorated_by.'
    return func
7

The decorator in the book assumes that its argument will already have some docstring, and it appends a string to that value. It does not expect to create a docstring for a function that lacks one.

1 Like