[Python-ideas] Statically checking purity of functions

[Christoph Groth]

Hello,

As far as I can tell the following has not been discussed yet on 
the searchable internet.  It may seem that my idea concerns only 
mypy, but since "typing" is part of the stdlib, I believe it is 
appropriate for this mailing list.

Even though Python is a very dynamic language, in practice many 
routines (when called with arguments of the intended type) are 
pure in the functional programming sense, i.e. they have no 
observable side effects and the result reproducibly depends on the 
arguments.

I think there could be many interesting applications of knowing 
whether a function is pure or not.  Example: Long-running 
computations often need to be parallelized.  There are plenty of 
ways to parallelize Python code (think of concurrent.futures or 
ipyparallel) and most of them make implicit assumptions that some 
functions are pure.

Another example: if the purity of functions would be known, 
Jupyter notebooks could automatically deduce the side effects of 
code cells and thus calculate their dependencies on each 
other. Changes to a cell could thus invalidate other (dependent) 
cells.

With plain Python (without type annotations) there is no way to 
verify the purity of a function as has been well explained here: 
http://stackoverflow.com/a/31664102

But couldn't this be done with a static type checker like mypy? If 
mypy would track functions as pure or not pure, it should be able 
to verify the claimed purity of any new function.  For example, 
the following function would pass the test,

@pure
def add(a: int, b: int) -> int:
   return a + b

while the next one would produce an error

@pure
def add_random(a: int) -> int:
   return a + random.randrange(10)

because the static checker would know that random.randrange() is 
not pure (it modifies the state of the random module).

Am I overseeing any problems with the above?

Christoph
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