How to print to stderr in Python? – Dev

The best answers to the question “How to print to stderr in Python?” in the category Dev.

QUESTION:

There are several ways to write to stderr:

# Note: this first one does not work in Python 3
print >> sys.stderr, "spam"

sys.stderr.write("spam\n")

os.write(2, b"spam\n")

from __future__ import print_function
print("spam", file=sys.stderr)

That seems to contradict zen of Python #13 , so what’s the difference here and are there any advantages or disadvantages to one way or the other? Which way should be used?

There should be one — and preferably only one — obvious way to do it.

ANSWER:

import sys
sys.stderr.write()

Is my choice, just more readable and saying exactly what you intend to do and portable across versions.

Edit: being ‘pythonic’ is a third thought to me over readability and performance… with these two things in mind, with python 80% of your code will be pythonic. list comprehension being the ‘big thing’ that isn’t used as often (readability).

ANSWER:

I found this to be the only one short, flexible, portable and readable:

# This line only if you still care about Python2
from __future__ import print_function

import sys

def eprint(*args, **kwargs):
    print(*args, file=sys.stderr, **kwargs)

The optional function eprint saves some repetition. It can be used in the same way as the standard print function:

>>> print("Test")
Test
>>> eprint("Test")
Test
>>> eprint("foo", "bar", "baz", sep="---")
foo---bar---baz

ANSWER:

Nobody’s mentioned logging yet, but logging was created specifically to communicate error messages. Basic configuration will set up a stream handler writing to stderr.

This script:

# foo.py
import logging

logging.basicConfig(format="%(message)s")
log = logging.getLogger(__name__)
log.warning('I print to stderr by default')
print('hello world')

has the following result when run on the command line:

$ python3 foo.py > bar.txt
I print to stderr by default

and bar.txt will contain the ‘hello world’ printed on stdout.

ANSWER:

Python 2:

print >> sys.stderr, "fatal error"

Python 3:

print("fatal error", file=sys.stderr)

Long answer

print >> sys.stderr is gone in Python3.
http://docs.python.org/3.0/whatsnew/3.0.html says:

Old: print >> sys.stderr, "fatal error"
New: print("fatal error", file=sys.stderr)

For many of us, it feels somewhat unnatural to relegate the destination to the end of the command. The alternative

sys.stderr.write("fatal error\n")

looks more object oriented, and elegantly goes from the generic to the specific. But note that write is not a 1:1 replacement for print.