Question

What are common uses for Python’s built-in coerce function? I can see applying it if I do not know the type of a numeric value as per the documentation, but do other common usages exist? I would guess that coerce() is also called when performing arithmetic computations, e.g. x = 1.0 +2. It’s a built-in function, so presumably it has some potential common usage?

Answer 1

Its a left over from early python, it basically makes a tuple of numbers to be the same underlying number type e.g.

>>> type(10)
<type 'int'>
>>> type(10.0101010)
<type 'float'>
>>> nums = coerce(10, 10.001010)
>>> type(nums[0])
<type 'float'>
>>> type(nums[1])
<type 'float'>

It is also to allow objects to act like numbers with old classes
(a bad example of its usage here would be …)

>>> class bad:
...     """ Dont do this, even if coerce was a good idea this simply
...         makes itself int ignoring type of other ! """
...     def __init__(self, s):
...             self.s = s
...     def __coerce__(self, other):
...             return (other, int(self.s))
... 
>>> coerce(10, bad("102"))
(102, 10)