Issues trying to read a datafile created with Python array module

Question

I have a data file with 64-bit IEEE floating point data. I created it with the Python array module:

float_array = array('d', data_list)
float_array.tofile(out_fname)

where data_list is a simple Python list containing integers.

Now I want to open that file and read from it. I tried this:

fname = 'C:\Data_Files\Python64_Float'
file_object  = open(fname, "rb")

I tried reading it two ways:

data = struct.unpack('f', file_object.read(4))

AND

data = file_object.read()

but in both cases it returns an array of zeroes, which is not what the file contains. I can open and read this file with another tool, but I need to read it in Python.

I also tried reading this with array.fromfile, but Visual Studio says "module 'array' has no attribute 'fromfile' -- but according to the Python site on the array module, it does have a fromfile attribute.

So my question is: how can I read a file created by the Python array module as 64-bit float?

Thanks for any help.

Anwarvic · Accepted Answer · 2020-05-16 20:28:49Z

1

You need to write your list as bytes into a file, then use array.frombytes method to read it.

Here is a full example:

Writing

import array

data_list = [1, 2, 3, 4]
float_array = array.array('d', data_list)
# write float_array as bytes
with open("Python64_Float", "wb") as fout:
    float_array.tofile(fout)

Reading

Now, use array.frombytes to read it just like so:

import array

another_float_array = array.array('d', [])
with open("Python64_Float", "rb") as fin:
    another_float_array.frombytes(fin.read())
print(another_float_array)
#array('d', [1.0, 2.0, 3.0, 4.0])

answered May 16, 2020 at 20:28

Anwarvic

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1 Comment

Anwarvic Over a year ago

Glad I could help !

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