How to split a single file into multiple file of different size using Python

First, I would suggest to convert your list into a list of 2-tuples, and use integers for the numbers instead of strings. Using it is easier that way. I'm using a list instead of a dict because a list has an order, and a dictionary hasn't.

fragments = [('IdleChk_1_E1.txt', 749), 
             ('IdleChk_2_E1.txt', 749),
             ('reg_fifo_E1.txt', 5922),
             ('igu_fifo_E1.txt', 161),
             ('protection_override_E1.txt', 1904),
             ('fw_asserts_E1.txt', 708),
             ('McpTrace.txt', 15578),
             ('phy_dump.txt', 129),
             ('GrcDumpE1.bin', 3629656)]

Then we open the file in binary mode (I'm using Python 3 here), read the required amount of data and write it to the output files.

with open('inputfile', 'rb') as inf:
    for fn, count in fragments:
        with open(fn, 'wb') as outf:
            outf.write(inf.read(count))

It would be a good idea to check that the sum of all fragment sizes is not greater than the file size. Or you could use -1 for the size of the last fragment, that would make read get all remaining data.

It's fairly obvious from your attempted solution that you are new to Python, but you've actually made quite surprising progress to make use of the subprocess library, so I am sure that you'll find that you will do better as time goes by. Often a problem appears difficult because you simply aren't aware of all the available features of the tools available. In this case it seems you are using head because you know that can be forced to do what you want, but I'm sure you'll agree it's not a comfortable solution.

It's difficult to deal with any procedure that takes nineteen arguments - the commands become rather difficult to understand, and it's much easier to make errors in writing them. A data-driven approach, where you describe in a text file how you want your files to be split, is likely to be more tractable. Then you can write a program that reads that description and makes use of it to split the file. Since Python can read and write files quite easily, you should then find there's no need to use shell scripting at all, which will make your solution much more portable.

If I've understood your shell script correctly, each head command takes a certain number of bytes from the file named in the nineteenth(!) argument and writes them out to a nominated file. So you might use a data file layout that contains lines of the form

N filename

where N is the number of lines to beo allow me to test this I created the following file in task_description.txt.

10 file1.txt
20 file2.txt
30 file3.txt

Like your program (if I've got it right) any bytes after the sixty specified will be ignored. So now I can write a program so15.py that reads the task description and processes some data file, named in it first command-line argument, accordingly:

import sys
in_file = sys.argv[1]
with open("task_description.txt") as td, open(in_file, "rb") as inf:
    for line in td:
        n, file_name = line.split()
        with open(file_name, "wb") as out_file:
            out_file.write(inf.read(int(n)))
        print("Wrote", n, "bytes to", file_name)

I then ran this using a data file that had over 60 bytes in it - the Misc/NEWS file from the Python distribution - using the command

python so15.py /Users/sholden/Projects/Python/cpython/Misc/NEWS

It gave the output

Wrote 10 bytes to file1.txt
Wrote 20 bytes to file2.txt
Wrote 30 bytes to file3.txt

As a check I then ran the command

wc -l file*.txt

with the following result

   0       1      10 file1.txt
   2       4      20 file2.txt
   2       6      30 file3.txt
   4      11      60 total

Hopefully you will be able to adapt this to solve your problem fairly easily.