TypeError: a bytes-like object is required, not 'str' when writing to a file in Python3

Question

I've very recently migrated to Py 3.5. This code was working properly in Python 2.7:

with open(fname, 'rb') as f:     lines = [x.strip() for x in f.readlines()]  for line in lines:     tmp = line.strip().lower()     if 'some-pattern' in tmp: continue     # ... code 

After upgrading to 3.5, I'm getting the:

TypeError: a bytes-like object is required, not 'str' 

error on the last line (the pattern search code).

I've tried using the .decode() function on either side of the statement, also tried:

if tmp.find('some-pattern') != -1: continue 

- to no avail.

I was able to resolve almost all 2:3 issues quickly, but this little statement is bugging me.

Answer 1

You opened the file in binary mode:

with open(fname, 'rb') as f: 

This means that all data read from the file is returned as bytes objects, not str. You cannot then use a string in a containment test:

if 'some-pattern' in tmp: continue 

You'd have to use a bytes object to test against tmp instead:

if b'some-pattern' in tmp: continue 

or open the file as a textfile instead by replacing the 'rb' mode with 'r'.