TypeError: a bytes-like object is required, not 'str' when writing to a file in Python 3


I’ve very recently migrated to Python 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’

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

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

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

– to no avail.

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

Asked By: masroore



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'.

Answered By: Martijn Pieters

For this small example, adding the only b before
'GET http://www.py4inf.com/code/romeo.txt HTTP/1.0nn' solved my problem:

import socket

mysock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
mysock.connect(('www.py4inf.com', 80))
mysock.send(b'GET http://www.py4inf.com/code/romeo.txt HTTP/1.0nn')

while True:
    data = mysock.recv(512)
    if (len(data) < 1):
    print (data);


What does the 'b' character do in front of a string literal?

Answered By: starter

Like it has been already mentioned, you are reading the file in binary mode and then creating a list of bytes. In your following for loop you are comparing string to bytes and that is where the code is failing.

Decoding the bytes while adding to the list should work. The changed code should look as follows:

with open(fname, 'rb') as f:
    lines = [x.decode('utf8').strip() for x in f.readlines()]

The bytes type was introduced in Python 3 and that is why your code worked in Python 2. In Python 2 there was no data type for bytes:

>>> s=bytes('hello')
>>> type(s)
<type 'str'>
Answered By: Suresh

You can encode your string by using .encode()


'Hello World'.encode()

As the error describes, in order to write a string to a file you need to encode it to a byte-like object first, and encode() is encoding it to a byte-string.

You have to change from wb to w:

def __init__(self):
    self.myCsv = csv.writer(open('Item.csv', 'wb')) 
    self.myCsv.writerow(['title', 'link'])


def __init__(self):
    self.myCsv = csv.writer(open('Item.csv', 'w'))
    self.myCsv.writerow(['title', 'link'])

After changing this, the error disappears, but you can’t write to the file (in my case). So after all, I don’t have an answer?

Source: How to remove ^M

Changing to ‘rb’ brings me the other error: io.UnsupportedOperation: write

Answered By: meck373

Try opening your file as text:

with open(fname, 'rt') as f:
    lines = [x.strip() for x in f.readlines()]

Additionally, here is a link for Python 3.x on the official page:
io — Core tools for working with streams.

And this is the open function: open

If you are really trying to handle it as a binary then consider encoding your string.

Answered By: Fernando D Jaime

You opened the file in binary mode:

The following code will throw
a TypeError: a bytes-like object is required, not ‘str’.

for line in lines:
    print(type(line))# <class 'bytes'>
    if 'substring' in line:

The following code will work – you have to use the decode() function:

for line in lines:
    line = line.decode()
    print(type(line))# <class 'str'>
    if 'substring' in line:
Answered By: Matan Hugi

Use the encode() function along with the hardcoded string value given in a single quote.


file.write(answers[i] + 'n'.encode())


line.split(' +++$+++ '.encode())
Answered By: Shiv Buyya

I got this error when I was trying to convert a char (or string) to bytes, the code was something like this with Python 2.7:

# -*- coding: utf-8 -*-

This is the way of Python 2.7 when dealing with Unicode characters.

This won’t work with Python 3.6, since bytes require an extra argument for encoding, but this can be little tricky, since different encoding may output different result:

print(bytes('ò', 'iso_8859_1')) # prints: b'xf2'
print(bytes('ò', 'utf-8')) # prints: b'xc3xb2'

In my case I had to use iso_8859_1 when encoding bytes in order to solve the issue.

Answered By: Ibrahim.H
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