I have configured logger to print both onto terminal stdout and to a file so I can have an archive of logging messages that I can refer to.
That is easily accomplished by adding a FileHandler to your logging object. Easy peasy.
What I want to accomplish now is to make argparse log also to the same file along with logs to stdout when it encounters parsing errors. So far it only prints to stdout. I looked in the argparse documentation but I can't find anything about setting a different output stream or pipe for argparse.
Is it possible to do? How?
Looking at the argparse.py source code there doesn't seem to be a way to configure this behaviour.
My suggestion(s) would be:
File a bug report with a patch :)
Override/patch:
print_* method(s)
error method.
The print_* method(s) seem to take an optional file argument which defaults to _sys.stdout.
Update: Alternatively you could do something like this whereby you redirect sys.stdout temporarily while you parse arguments:
from contextlib import contextmanager
#contextmanager
def redirect_stdout_stderr(stream):
old_stdout = sys.stdout
old_stderr = sys.stderr
sys.stdout = stream
sys.stderr = stream
try:
yield
finally:
sys.stdout = old_stdout
sys.stderr = old_stderr
with redirct_stdout_stderr(logstream):
args = parser.parse_args()
There seems to be no way to do this through the API.
However, you can do the following:
class LoggingArgumentParser(argparse.ArgumentParser):
"""Custom ArgumentPaarser that overrides _print_message"""
def _print_message(self, message, file=None):
if message:
logger.write(message)
While the answer given by #James Mills is great and solves the issue, there is no need for a generator in this case. Hence, the yield is redundant. Another way of achieving the same (without the generator) would be to write your own context manager without using the inbuilt contextlib.contextmanager decorator. Like the following.
class redirect_stdout_stderr(object):
def __init__(self, stream):
# Save the old std streams
self.old_stream = sys.stdout
self.old_error_stream = sys.stderr
self.fstream = stream
def __enter__(self):
# Change the std streams to your streams when entering
sys.stdout = self.fstream
sys.stderr = self.fstream
def __exit__(self, exc_type, exc_value, exc_traceback):
# Change the std streams back to the original streams while exiting
sys.stdout = self.old_stream
sys.stderr = self.old_error_stream
In your case you can do something as follows.
with redirect_stdout_stderr(logstream):
# __enter__() is executed
args = parser.parse_args()
# __exit__() is executed
Hope this helps!
Related
Background
I have a very large python application that launches command-line utilities to get pieces of data it needs. I currently just redirect the python launcher script to a log file, which gives me all of the print() output, plus the output of the command-line utilities, i.e.:
python -m launcher.py &> /root/out.log
Problem
I've since implemented a proper logger via logging, which lets me format the logging statements more precisely, lets me limit log file size, etc. I've swapped out most of my print()statements with calls to my logger. However, I have a problem: none of the output from the command-line applications is appearing in my log. It instead gets dumped to the console. Also, the programs aren't all launched the same way: some are launched via popen(), some by exec(), some by os.system(), etc.
Question
Is there a way to globally redirect all stdout/stderr text to my logging function, without having to re-write/modify the code that launches these command-line tools? I tried setting setting the following which I found in another question:
sys.stderr.write = lambda s: logger.error(s)
However it fails with "sys.stderr.write is read-only".
While this is not a full answer, it may show you a redirect to adapt to your particular case. This is how I did it a while back. Although I cannot remember why I did it this way, or what the limitation was I was trying to circumvent, the following is redirecting stdout and stderr to a class for print() statements. The class subsequently writes to screen and to file:
import os
import sys
import datetime
class DebugLogger():
def __init__(self, filename):
timestamp = datetime.datetime.strftime(datetime.datetime.utcnow(),
'%Y-%m-%d-%H-%M-%S-%f')
#build up full path to filename
logfile = os.path.join(os.path.dirname(sys.executable),
filename + timestamp)
self.terminal = sys.stdout
self.log = open(logfile, 'a')
def write(self, message):
timestamp = datetime.datetime.strftime(datetime.datetime.utcnow(),
' %Y-%m-%d-%H:%M:%S.%f')
#write to screen
self.terminal.write(message)
#write to file
self.log.write(timestamp + ' - ' + message)
self.flush()
def flush(self):
self.terminal.flush()
self.log.flush()
os.fsync(self.log.fileno())
def close(self):
self.log.close()
def main(debug = False):
if debug:
filename = 'blabla'
sys.stdout = DebugLogger(filename)
sys.stderr = sys.stdout
print('test')
if __name__ == '__main__':
main(debug = True)
import sys
import io
class MyStream(io.IOBase):
def write(self, s):
logger.error(s)
sys.stderr = MyStream()
print('This is an error', stream=sys.stderr)
This make all call to sys.stderr go to the logger.
The original one is always in sys.__stderr__
I'm importing a module foo which uses Python's logging module. However, foo produces a huge amount of logging output, and I need to use stdout to communicate important information to the user, which is largely being drowned out by the ridiculous output of the module I'm importing.
How can I disable the module's ability to log to stdout without modifying foo's code? I still want it to log to the files it logs to, but I don't want it logging to stdout.
I have tried the following:
logging.getLogger("foo").propagate = False
and
#contextlib.contextmanager
def nostdout():
class DummyFile(object):
def write(self, x): pass
save_stdout = sys.stdout
sys.stdout = DummyFile()
yield
sys.stdout = save_stdout
with nostdout(): import foo
Try the following:
logging.getLogger(<logger_name_used_in_foo>).propagate = False
I'm referencing this article.
In general, if you want to capture anything written to stdout you can use the contextlib in Python 3:
from contextlib import redirect_stdout
f = io.StringIO()
with redirect_stdout(f):
print('foobar')
call_annoying_module()
print('Stdout: "{0}"'.format(f.getvalue()))
On Python 3.4 and older, redirect_stdout can be implemented like this:
from contextlib import contextmanager
#contextmanager
def stdout_redirector(stream):
old_stdout = sys.stdout
sys.stdout = stream
try:
yield
finally:
sys.stdout = old_stdout
If the library has any C bindings that print using puts then it gets more complicated. See the article.
The easiest case is when you're running another program using subprocess,
then all stdout output can be easily captured.
proc = subprocess.Popen("echo output".split(), stdout=subprocess.PIPE)
std_output, err_output = proc.communicate()
I was wondering if this is possible in python:
# module1
def test():
print('hey')
# module2
import module1
module1.test() # prints to stdout
Without modifying module1 is there any way to wrap this in module2 so that I can capture the
print('hey') inside a variable? Apart from running module1 as a script?
I don't want to be responsible for modifying sys.stdout and then restoring it to its previous values. The above answers don't have any finally: clause, which can be dangerous integrating this into other important code.
https://docs.python.org/3/library/contextlib.html
import contextlib, io
f = io.StringIO()
with contextlib.redirect_stdout(f):
module1.test()
output = f.getvalue()
You probably want the variable output which is <class 'str'> with the redirected stdout.
Note: this code is lifted from the official docs with trivial modifications (but tested). Another version of this answer was already given to a mostly duplicated question here: https://stackoverflow.com/a/22434594/1092940
I leave the answer here because it is a much better solution than the others here IMO.
Yes, all you need is to redirect the stdout to a memory buffer that complies with the interface of stdout, you can do it with StringIO. This works for me in 2.7:
import sys
import cStringIO
stdout_ = sys.stdout #Keep track of the previous value.
stream = cStringIO.StringIO()
sys.stdout = stream
print "hello" # Here you can do whatever you want, import module1, call test
sys.stdout = stdout_ # restore the previous stdout.
variable = stream.getvalue() # This will get the "hello" string inside the variable
Yes, you can. You need to take control of sys.stdout. Something like this:
import sys
stdout_ = sys.stdout #Keep track of the previous value.
sys.stdout = open('myoutputfile.txt', 'w') # Something here that provides a write method.
# calls to print, ie import module1
sys.stdout = stdout_ # restore the previous stdout.
For Python 3:
# redirect sys.stdout to a buffer
import sys, io
stdout = sys.stdout
sys.stdout = io.StringIO()
# call module that calls print()
import module1
module1.test()
# get output and restore sys.stdout
output = sys.stdout.getvalue()
sys.stdout = stdout
print(output)
There's No need to use another module, just class object with write attribute, with one input, which you can save in another variable. for ecample
CLASS:
class ExClass:
def __init__(self):
self.st = ''
def write(self, o): #here o is the output that goes to stdout
self.st += str(o)
MAIN Program:
import sys
stdout_ = sys.stdout
var = ExClass()
sys.stdout = var
print("Hello") # these will not be pronted
print("Hello2") # instead will be written in var.st
sys.stdout = stdout_
print(var.st)
output will be
Hello
Hello2
Sending ftplib debug output to the logging module
Based on the approach taken by jimmy kumar ahalpara answer, I was able to capture ftplib's debug output into logging. ftplib was around before the logging module and uses print to emit debug messages.
I'd tried reassigning the print function to a logging method but I couldn't get that to work. The code below works for me.
I should think this will work with other modules as well but there would not be any granularity between different module's output as it's capturing everything sent to stdout to the same logger.
# convenience class to redirect stdout to logging
class SendToLog:
def __init__(self, logging_method):
self.logger = logging
_method
def write(self, o):
if str(o).strip(): # ignore empty lines
self.logger(str(o))
import logging
import sys
# code to initialise logging output and handlers ...
# ...
# get logger for ftplib and redirect it's print output to our log
ftp_logger = logging.getLogger('ftplib')
# note: logging's debug method is passed to the class, the instance then calls this method
sys.stdout = SendToLog(ftp_logger.debug)
# code to do stuff with ftplib ...
# remember to set ftplib's debug level > 0 or there will be no output
# FTP.set_debuglevel(1)
# ...
# important to finalise logging and restore stdout
logging.shutdown()
sys.stdout = sys.__stdout__
python3 stdout ftplib logging
I have this somewhat complicated command line function in Python (lets call it myFunction()), and I am working to integrate it in a graphical interface (using PySide/Qt).
The GUI is used to help select inputs, and display outputs. However, myFunction is designed to work as a stand-alone command line function, and it occasionnaly prints out the progress.
My question is: how can I intercept these print calls and display them in the GUI?
I know it would be possible to modify myFunction() to send processEvents() to the GUI, but I would then lose the ability to execute myFunction() in a terminal.
Ideally, I would like something similar to Ubuntu's graphical software updater, which has a small embeded terminal-looking widget displaying what apt-get would display were it executed in a terminal.
you could redirect stdout and restore after. for example:
import StringIO
import sys
# somewhere to store output
out = StringIO.StringIO()
# set stdout to our StringIO instance
sys.stdout = out
# print something (nothing will print)
print 'herp derp'
# restore stdout so we can really print (__stdout__ stores the original stdout)
sys.stdout = sys.__stdout__
# print the stored value from previous print
print out.getvalue()
Wrap it with a function that hijacks stdout:
def stdin2file(func, file):
def innerfunc(*args, **kwargs):
old = sys.stdout
sys.stdout = file
try:
return func(*args, **kwargs)
finally:
sys.stdout = old
return innerfunc
Then simply provide a file like object that supports write():
class GUIWriter:
def write(self, stuff):
#send stuff to GUI
MyFunction = stdin2file(MyFunction, GUIWriter())
The wrapper can be turned into a decorator too:
def redirect_stdin(file):
def stdin2file(func, file):
def innerfunc(*args, **kwargs):
old = sys.stdout
sys.stdout = file
try:
return func(*args, **kwargs)
finally:
sys.stdout = old
return innerfunc
return stdin2file
The use it when declaring MyFunction():
#redirect_stdin(GUIWriter())
def MyFunction(a, b, c, d):
# any calls to print will call the 'write' method of the GUIWriter
# do stuff
Here is a Python 3 pattern using contextmanager that both encapsulates the monkey-patch technique and also ensures that sys.stdout is restored in case of an exception.
from io import StringIO
import sys
from contextlib import contextmanager
#contextmanager
def capture_stdout():
"""
context manager encapsulating a pattern for capturing stdout writes
and restoring sys.stdout even upon exceptions
Examples:
>>> with capture_stdout() as get_value:
>>> print("here is a print")
>>> captured = get_value()
>>> print('Gotcha: ' + captured)
>>> with capture_stdout() as get_value:
>>> print("here is a print")
>>> raise Exception('oh no!')
>>> print('Does printing still work?')
"""
# Redirect sys.stdout
out = StringIO()
sys.stdout = out
# Yield a method clients can use to obtain the value
try:
yield out.getvalue
finally:
# Restore the normal stdout
sys.stdout = sys.__stdout__
All printing is done via sys.stdout, which is a ordinary file-like object: iirc, it requires a method write(str). As long as your replacement has that method, it's quite easy to drop in your hook:
import sys
class CaptureOutput:
def write(self, message):
log_message_to_textbox(message)
sys.stdout = CaptureOutput()
The actual contents of log_message_to_textbox are up to you.
As part of trying to test a legacy function's 'print to stdout' side-effect, I want to capture stdout for later replay. I use mock.
goals (fulfill as many as possible!)
stdout still prints where it normally would, but there is an additional recorder
ideally, this should be 'patched' or only occur in a context
My implementation (below) has patching that seems a bit heavy / gross. Is there a saner way to do it? cStringIO? Any better parts of mock I can use, rather that my __getattr__ hack?
class StreamCapturing(object):
def __init__(self, stream):
self.captured = []
self.stream = stream
def __getattr__(self,attr):
return getattr(self.stream,attr)
def write(self, data):
self.captured.append(data)
self.stream.write(data)
import sys
import mock
with mock.patch('sys.stdout',StreamCapturing(sys.stdout)) as ctx:
sys.stdout.write('a\n')
print 'stdout'
sys.__stdout__.write("the real one\n")
print sys.stdout.captured
sys.stdout.flush()
assert getattr(sys.stdout,'captured') is None
You don't even need to save the previous stdout python does it for you and yes use cStringIO
import sys
from cStringIO import StringIO
sys.stdout = captured = StringIO()
print "test string"
# test stuff
captured = captured.getvalue()
sys.stdout = sys.__stdout__
print "captured",captured
You do not need mock in this situation:
saved_stdout = sys.stdout
sys.stdout = StreamCapturing(saved_stdout)
print "stdout"
captured = "".join(sys.stdout.captured)
sys.stdout=saved_stdout
print "captured: ", captured