[Python] Why is my Python parser method returning empty strings?

I am trying to write a simple parser method in Python. It takes in a filename pointing to a file of a certain format. An example of this format is below:

File:
.type = INFILE
.fmt = CFP_INPUTFILE_FMT_2
Data:
.cases
.given = True
.numCases = 2
.case
.numlines = 2
.line
.value = 3
.value = 3
.line
.value = 3
.value = 3
.case
.line
.value = 3
.value = 3
.line
.value = 3
.value = 3


The parser reads this file and outputs the raw data to a terminal via subprocess.run() and prints the output. The data printed for the file should look like this:

2
33
33
33
33


The method itself looks something like this:

@classmethod
def input_file_fmt_1_to_input(cls, inputfile: str):
with open('temp.txt', 'w') as temp:
try:
with open(inputfile, 'r') as file:
case = 0
line = 0
case1line1 = ''
case1line2 = ''
case2line1 = ''
case2line2 = ''
for lne in file.readlines():
cleanline = lne.lstrip().rstrip()
if cleanline.startswith('.numCases'):
split = cleanline.split(' ')
value = str(split[2]) + '\n'
temp.write(value)
elif cleanline.startswith('.case'):
case += 1
elif cleanline.startswith('.line'):
line += 1
elif cleanline.startswith('.value') and case == 1 and line == 1:
case1line1 = case1line1 + str(cleanline.split(' ')[2]) + ' '
elif cleanline.startswith('.value') and case == 1 and line == 2:
case1line2 = case1line2 + str(cleanline.split(' ')[2]) + ' '
elif cleanline.startswith('.value') and case == 2 and line == 1:
case2line1 = case2line1 + str(cleanline.split(' ')[2]) + ' '
elif cleanline.startswith('.value') and case == 2 and line == 2:
case2line2 = case2line2 + str(cleanline.split(' ')[2]) + ' '
else:
pass

if case1line1 != '':
temp.write(case1line1)
temp.write('\n')
elif case1line2 != '':
temp.write(case1line2)
temp.write('\n')
elif case1line3 != '':
temp.write(case2line1)
temp.write('\n')
elif case1line4 != '':
temp.write(case2line2)
temp.write('\n')

result = run('cat temp.txt', shell=True, capture_output=True)
print(result.stdout)

except FileNotFoundError as e:
raise CfpRuntimeError from e


I've written a pytest test for this method, which looks like this:

def test_input_file_fmt_1_to_input_test(capsys):
cfp_testcontext.InputParser.input_file_fmt_1_to_input('../../TEST/modules_test/testfile.cfpin')
captured = capsys.readouterr()
assert captured.out == '2\n3 3 \n3 3 \n3 3 \n3 3 '


When I run the test, I get an assertion error:

assert "b''\nb''\nb''\nb''\n" == "2\n3 3 \n3 3 \n3 3 \n3 3 "

1. Where are the strings are being transformed into byte strings?
2. Why does my result contain only empty strings?

I have tried writing a a fake method and fake test using capsys to see if the output was empty, and if it was a byte string. The output was a byte string, so I believe that the conversion is happening from within pytest. However, the function, which wrote 'hello' to the console, worked perfectly, so I am pretty sure there is something wrong with my logic.

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