Python 3: How to specify stdin encoding

Question

While porting code from Python 2 to Python 3, I run into this problem when reading UTF-8 text from standard input. In Python 2, this works fine:

for line in sys.stdin:
    ...

But Python 3 expects ASCII from sys.stdin, and if there are non-ASCII characters in the input, I get the error:

UnicodeDecodeError: 'ascii' codec can't decode byte .. in position ..: ordinal not in range(128)

For a regular file, I would specify the encoding when opening the file:

with open('filename', 'r', encoding='utf-8') as file:
    for line in file:
        ...

But how can I specify the encoding for standard input? Other SO posts (e.g. How to change the stdin encoding on python) have suggested using

input_stream = codecs.getreader('utf-8')(sys.stdin)
for line in input_stream:
    ...

However, this doesn't work in Python 3. I still get the same error message. I'm using Ubuntu 12.04.2 and my locale is set to en_US.UTF-8.

Answer 1

Python 3 does not expect ASCII from sys.stdin. It'll open stdin in text mode and make an educated guess as to what encoding is used. That guess may come down to ASCII, but that is not a given. See the sys.stdin documentation on how the codec is selected.

Like other file objects opened in text mode, the sys.stdin object derives from the io.TextIOBase base class; it has a .buffer attribute pointing to the underlying buffered IO instance (which in turn has a .raw attribute).

Wrap the sys.stdin.buffer attribute in a new io.TextIOWrapper() instance to specify a different encoding:

import io
import sys

input_stream = io.TextIOWrapper(sys.stdin.buffer, encoding='utf-8')

Alternatively, set the PYTHONIOENCODING environment variable to the desired codec when running python.

From Python 3.7 onwards, you can also reconfigure the existing std* wrappers, provided you do it at the start (before any data has been read):

# Python 3.7 and newer
sys.stdin.reconfigure(encoding='utf-8')