Why is multiprocessing.apply_async unpacking a string as arguments?

I was trying to do some multiprocessing.

import multiprocessing

with multiprocessing.Pool() as pool:
  pool.apply_async(func,('some string'))

I get an error:

TypeError: func() takes 1 positional arguments but 261 were given

This is very suspicious because the string I was using as the argument had 260 characters.

Does anyone know why this is happening and how to solve it?

Solution

According to Python docs (emphasis mine!),

A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective.

So, the expression ('a') is actually creating a string of value 'a', but the expression ('a',) is creating a tuple of a single string element 'a'.

That said, you can refactor your code as

import multiprocessing

with multiprocessing.Pool() as pool:
  pool.apply_async(func, ('some string',))