Reading large compressed files in python

This might be a simple question but I can't seem to find the answer to this or why it is not working on this specific case.

I want to read large files, they can be compressed or not. I used contextlib to write a contextmanager function to handle this. Then using the with statement I read the files in the main script.

My problem here is that the script uses a lot of memory then get's killed (testing using a compressed file). What am I doing wrong? Should I approach this differently?

def process_vcf(location):
    logging.info('Processing vcf')
    logging.debug(location)
    with read_compressed_or_not(location) as vcf:
        for line in vcf.readlines():
            if line.startswith('#'):
                logging.debug(line)

@contextmanager
def read_compressed_or_not(location):
    if location.endswith('.gz'):
        try: 
            file = gzip.open(location)
            yield file
        finally:
            file.close()
    else:
        try: 
            file = open(location, 'r')
            yield file
        finally:
            file.close()

source https://stackoverflow.com/questions/70159969/reading-large-compressed-files-in-python