jeremiahbuddha jeremiahbuddha - 25 days ago 22
Python Question

Suppress stdout / stderr print from Python functions

I have a Python script that is using some closed-box Python functions (i.e. I can't edit these functions) provided by my employer. When I call these functions, they are printing output to my linux terminal that I would like to suppress. I've tried redirecting stdout / stderr via;

orig_out = sys.stdout
sys.stdout = StringIO()
rogue_function()
sys.stdout = orig_out


but this fails to catch the output. I think the functions I'm calling via-Python (rogue_function() from above) are really wrappers for compiled C-code, which are actually doing the printing.

Does anyone know of a way I can do a "deep-capture" of any print handed to stdout / stderr by a function (and any sub-functions that function calls)?

UPDATE:

I ended up taking the method outlined in the selected answer below and writing a context manager to supress stdout and stderr:

# Define a context manager to suppress stdout and stderr.
class suppress_stdout_stderr(object):
'''
A context manager for doing a "deep suppression" of stdout and stderr in
Python, i.e. will suppress all print, even if the print originates in a
compiled C/Fortran sub-function.
This will not suppress raised exceptions, since exceptions are printed
to stderr just before a script exits, and after the context manager has
exited (at least, I think that is why it lets exceptions through).

'''
def __init__(self):
# Open a pair of null files
self.null_fds = [os.open(os.devnull,os.O_RDWR) for x in range(2)]
# Save the actual stdout (1) and stderr (2) file descriptors.
self.save_fds = (os.dup(1), os.dup(2))

def __enter__(self):
# Assign the null pointers to stdout and stderr.
os.dup2(self.null_fds[0],1)
os.dup2(self.null_fds[1],2)

def __exit__(self, *_):
# Re-assign the real stdout/stderr back to (1) and (2)
os.dup2(self.save_fds[0],1)
os.dup2(self.save_fds[1],2)
# Close the null files
os.close(self.null_fds[0])
os.close(self.null_fds[1])


To use this you just:

with suppress_stdout_stderr():
rogue_function()


This works "pretty good". It does suppress the printout from the rogue functions that were cluttering up my script. I noticed in testing it that it lets through raised exceptions as well as some logger print, and I'm not entirely clear why. I think it has something to do with when these messages get sent to stdout / stderr (I think it happens after my context manager exits). If anyone can confirm this, I'd be interested in hearing the details ...

Answer

This approach (found through the related sidebar) might work. It reassigns the file descriptors rather than just the wrappers to them in sys.stdout, etc.

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