I have already found the source for the numpy.ma.where() function but it seems to be calling the numpy.where() function and to better understand it I would like to take a look if possible.

Most Python functions are written in the Python language, but some functions are written in something more native (often the C language).

Regular Python functions ("pure Python")

There are a few techniques you can use to ask Python itself where a function is defined. Probably the most portable uses the inspect module:

>>> import numpy
>>> import inspect
>>> inspect.isbuiltin(numpy.ma.where)
False
>>> inspect.getsourcefile(numpy.ma.where)
'.../numpy/core/multiarray.py'

But this won't work with a native ("built-in") function:

>>> import numpy
>>> import inspect
>>> inspect.isbuiltin(numpy.where)
True
>>> inspect.getsourcefile(numpy.where)
TypeError: <built-in function where> is not a module, class, method, function, traceback, frame, or code object

Native ("built-in") functions

Unfortunately, Python doesn't provide a record of source files for built-in functions. You can find out which module provides the function:

>>> import numpy as np
>>> np.where
<built-in function where>
>>> np.where.__module__
'numpy.core.multiarray'

Python won't help you find the native (C) source code for that module, but in this case it's reasonable to look in the numpy project for C source that has similar names. I found the following file:

numpy/core/src/multiarray/multiarraymodule.c

And in that file, I found a list of definitions (PyMethodDef) including:

    {"where",(PyCFunction)array_where,METH_VARARGS, NULL},

This suggests that the C function array_where is the one that Python sees as "where".

The array_where function is defined in the same file, and it mostly delegates to the PyArray_Where function.

In short

NumPy's np.where function is written in C, not Python. A good place to look is PyArray_Where.