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Not sure why I cannot find this, but what does the following mean?

hashvalue_byte_size = len(bytes(np.int64(42).data))

Going from left to right, I would generically say this describes the length of bytes in np.int64(42).data.

So the byte length is 8.

I think 'np.int64()' means an integer in (-9223372036854775808 to 9223372036854775807) or an unsigned integer in (0 to 18446744073709551615)

Or does the prefix 'np' make int64() something else?

Finally, what does '42' stand for? What does .data mean?

It makes most sense to parse an expression like this from the inside out:

In [189]: np.int64(42)
Out[189]: 42
In [190]: type(_)
Out[190]: numpy.int64
In [191]: np.int64(42).data
Out[191]: <memory at 0x7f7dc41a82e8>
In [192]: type(_)
Out[192]: memoryview
In [193]: np.int64(42)
Out[193]: 42
In [194]: type(_)
Out[194]: numpy.int64
In [195]: np.int64(42).data
Out[195]: <memory at 0x7f7dcc05cac8>
In [196]: type(_)
Out[196]: memoryview
In [197]: bytes(np.int64(42).data)
Out[197]: b'*x00x00x00x00x00x00x00'
In [198]: len(_)
Out[198]: 8

It might make more sense to look at a numpy array, with one or more elements:

In [204]: np.array(42)
Out[204]: array(42)
In [205]: _.dtype
Out[205]: dtype('int64')
In [206]: np.array(42).data
Out[206]: <memory at 0x7f7dcc054780>
In [207]: bytes(np.array(42).data)
Out[207]: b'*x00x00x00x00x00x00x00'
In [208]: bytes(np.array([42,43]).data)
Out[208]: b'*x00x00x00x00x00x00x00+x00x00x00x00x00x00x00'

The array object has attributes, and a databuffer. That buffer stores the data, in this case as an 8 byte integer for each element. The bytes(...data) just produces a bytestring representation of that buffer.

Same thing with the tobytes method:

In [209]: np.array([42,43]).tobytes()
Out[209]: b'*x00x00x00x00x00x00x00+x00x00x00x00x00x00x00'

When you try to decipher expressions like this one, you should not go from left to right, but instead from inside to outside. As you can see below, the first two statements define a numpy.int64 object which takes the value 42. It means that 42 is saved in memory as a 64 bits integer. The next call gives you the memory address of your object. Right after, I think you get the content of your memory address expressed in a language that I do not know. And the last one simply gives you the number of bytes which are allocated at the memory address. Here it is 8, as you have allocated space for a 64 bits integer (1 byte = 8 bits).

import numpy

print(numpy.int64(42))
# 42
print(type(numpy.int64(42)))
# <class 'numpy.int64'>
print(numpy.int64(42).data)
# <memory at 0x7f5e43221588>
print(bytes(numpy.int64(42).data))
# b'*x00x00x00x00x00x00x00'
print(len(bytes(numpy.int64(42).data)))
# 8