basic version of DNA FASTA records converted to RNA, see https://github.com/fomightez/sequencework/blob/master/ConvertSeq/ConvertFASTAdnaSEQtoRNA.py for a fancier version

DNA_to_RNAsimple.py

#! /usr/bin/env python

# DNA_to_RNA.py
# basic version of DNA FASTA records converted to RNA, see https://github.com/fomightez/sequencework/blob/master/ConvertSeq/ConvertFASTAdnaSEQtoRNA.py for a fancier version

# adapted from start and end of latlon_3.py - from Chapter 10 PCfB
# Read in each line of the example file


# Set the input file name
# (The program must be run from within the directory
#  that contains this data file)
InFileName = 'dna.faa'

# Open the input file for reading
InFile = open(InFileName, 'r')

# Initialize the counter used to keep track of records
FASTA_records_counter = 0

# Open the output file for writing
# Do this *before* the loop, not inside it
OutFileName=InFileName + "_rna.faa"

OutFile=open(OutFileName,'w') # You can append instead with 'a'

# Loop through each line in the file
for line in InFile:
        line = line.strip ();#this way I have better knowledge of first character and control of ends ultimately becaue I know there isn't any unless I add
        if line.startswith('>'): # only need those that start with the greater-than symbol
            FASTA_records_counter += 1  #<-- short way to write `FASTA_records_counter = FASTA_records_counter + 1`
            output_line = line
        else:
            output_line = (line.upper()).replace('T','U')

        OutFile.write(output_line +"\n")


# After the loop is completed, close the files
InFile.close()
OutFile.close()