What is the best way (standard) to display an error to the user in Python (for example: bad syntax, invalid arguments, logic errors)?
The method should print the error in the standard error and exit the program.
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2 Answers
In small programs, I use something like this:
import sys
def error(message):
sys.stderr.write("error: %s\n" % message)
sys.exit(1)
For bigger tools, I use the logging package.
def error(message):
logging.error('error: ', message)
sys.exit(1)
5 Comments
yassin
How can I use logging to output to standard error (I already use it to output to a file)? How can I separe both streams?
leoluk
logging writes to stderr by default; if you want to seperate the streams, look at this: docs.python.org/library/…
Joe D
How about
raise SystemExit instead of sys.exit, seems a bit more pythonic to me.
AndiDog
You forgot the newline in the
sys.stderr.write call.leoluk
thanks, newline added; sys.exits allows to specify a return value
In Python 2, for example:
import sys
print>>sys.stderr, "Found", numerrs, "errors in the input."
sys.exit(1)
In Python 3 (or 2.6 or 2.7 with the from __future__ import print_function statement at the top of your module), print becomes a function and the syntax for "printing to standard error" is now
print("Found", numerrs, "errors in the input.", file=sys.stderr)
(a somewhat more friendly syntax than the strange >> needed in Python 2;-).
The (small) advantage of print versus (say) sys.stderr.write is that you get "all the modern conveniences": all parts of the error message are automatically turned into strings, they're output with space separators, and a line-end is output for you at the end (unless you specifically request otherwise). To use sys.stderr.write, you need to build the exact string to output, typically using string-formatting constructs -- not a big deal, of course, just a small matter of convenience.
logging is usually preferred, but not when you specifically want the error message to go to standard error, and to standard error only: logging offers many, many more possibilities (send messages of various severity, filter some, put some to files, and so on), but therefore it's inevitably a little bit more complex.
answered Sep 11, 2010 at 17:52
Alex Martelli
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1 Comment
Philip Kearns
One up-vote for clarifying the purpose of
logging.