How can I initialize this ctypes Structure array?

Question

I have a structure defined using class type and I want to initialize test_arr like this:

from ctypes import *

class ST_DATA(Structure):
    _fields_ = [("test1", c_int),
                ("test2", c_double),
                ("test_arr", c_double*2)]

stmyData = ST_DATA(1, 2, (3, 4))
print(stmyData.test1, stmyData.test2, stmyData.test_arr)

Result:

1 2.0 <__main__.c_double_Array_2 object at 0x11BB2AD8>

I don't understand this situation. How can I initialize this structure array in Python?

Answer 1

It is initialized, but the default print representation doesn't reflect the data...just the object's type and address.

It's a good habit to write a debug representation for classes so printing a class instance is more convenient. In this case, ctypes arrays can be converted to lists to see their content:

from ctypes import *

class ST_DATA(Structure):

    _fields_ = [('test1', c_int),
                ('test2', c_double),
                ('test_arr', c_double * 2)]

    # debug representation "magic" method
    def __repr__(self):
        return f'ST_DATA(test1={self.test1}, test2={self.test2}, test_arr={list(self.test_arr)})'

stmyData = ST_DATA(1, 2, (3, 4))
print(stmyData)

Output:

ST_DATA(test1=1, test2=2.0, test_arr=[3.0, 4.0])

See also:

object.__repr__(self)

Called by the repr() built-in function to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned. The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an “informal” string representation of instances of that class is required.

This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.

object.__str__(self)

Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. The return value must be a string object.

This method differs from object.__repr__() in that there is no expectation that __str__() return a valid Python expression: a more convenient or concise representation can be used.

The default implementation defined by the built-in type object calls object.__repr__().