We wouldn't write the code for you from scratch (thats your job), but its an easy problem because in Python any string is a "array" or "dictionary". Thus

bacteria1 = 'GRRKAAVARAYMTAGKGNVTVNKKDYKDFFPVPTLQYKITQPFAITETLGQYDVIEEXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
print (bacteria1[0:120])

This will define the first gene in your list, remembering the array starts at 0 whilst your gene starts at position 1 (so you need to subtract 1). You simply parse the input list so the gene lengths form a dictionary and loop through the dictionary of gene lengths ensuring the last length is added to the current one and then issuing a write command.

You don't really need biopython in this instance, unless it will parse the gene length file you need to open (I'm not aware it can).

BTW the format you supplied isn't fasta it looks like a Biopython statement, again in this problem I wouldn't bother because Python is perfectly capable of splicing a string.

We wouldn't write the code for you from scratch (thats your job), but its an easy problem because in Python any string is an "array" or "dictionary". Thus

bacteria1 = 'GRRKAAVARAYMTAGKGNVTVNKKDYKDFFPVPTLQYKITQPFAITETLGQYDVIEEXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
print (bacteria1[0:120])

This will define the first gene in your list, remembering the array starts at 0 whilst your gene starts at position 1 (so you need to subtract 1). You simply parse the input list so the gene lengths form a dictionary and loop through the dictionary of gene lengths ensuring the last length is added to the current one and then issuing a write command.

You don't really need biopython in this instance, unless it will parse the gene length file you need to open (I'm not aware it can).

BTW the format you supplied isn't fasta it looks like a Biopython statement, again in this problem I wouldn't bother because Python is perfectly capable of splicing a string.