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I'm editing some Python code containing the line:

arr = (c_double * len(self.static_mapped[0, :]))(*self.static_mapped[0, :])

where field self.static_mapped is created using numpy.zeros and where c_double is imported from ctypes.

Based on the context, I'm assuming this is typecasting from numpy doubles to C-type doubles, but this syntax doesn't appear in any of the Python guides I've consulted, and even my IDE is complaining that "'int' object is not callable".

Am I correct in my interpretation of its meaning? What sequence of function calls is going on under the hood? Is this a well-known code idiom in Python or is there a more standard/straightforward way of accomplishing this?

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Strictly from a syntactic perspective, your code should be equivalent to

t = c_double * len(self.static_mapped[0, :])
arr = t(*self.static_mapped[0, :])

This implies that c_double.__mul__ is overridden to produce some other type object that gets called to produce an array. No type-casting, just dynamic type generation.

Specifically, multiplying type by an integer n creates an array of n values:

>>> c_double * 2
<class '__main__.c_double_Array_2'>
>>> c_double * 3
<class '__main__.c_double_Array_3'>
>>> c_double * 7
<class '__main__.c_double_Array_7'>

Note that "type multiplication" is not associative:

# An array of arrays
>>> c_double * 7 * 2
<class '__main__.c_double_Array_7_Array_2'>

# A single array
>>> c_double * (7 * 2)
<class '__main__.c_double_Array_14'>
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Ah, I see! And the returned c_double_Array_X class has a constructor that accepts a variadic list of numpy double arguments. Got it. Thanks!

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