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Catches/silences stdout/stderr output.

Project description

Provides a context manager that catches/suppresses output from sys.stderr and sys.stdout (StdOutCatcher, StdErrCatcher). Also provides an easy way to gather both stdout and stderr from processes while optionally piping stdin to the process as a str or bytes.


StdOutCatcher / StdErrCatcher

StdOutCatcher(escaped=False, max_length=0)

This will suppress any output running through sys.stdout or sys.stderr, and save it in an attribute for possible future use.


  • escaped: If truthy, output is “encoded” using repr(), but without quotes. Default: False
  • max_length: If non-zero, final output will not exceed max_length. Once max_length is reached, further write() calls will not write to the original stream. Default: 0


from outputcatcher import StdErrCatcher, StdOutCatcher

# Catching stdout
with StdOutCatcher() as fakeout:
    print('This is a test. you shouldn\'t see it right away.')
print('Captured stdout: {}'.format(fakeout.output))

# Catching stderr
with StdErrCatcher() as fakeerr:
    print('Testing stderr output.', file=sys.stderr)
print('Captured stderr: {}'.format(fakeerr.output))


ProcessOutput(args, stdin_data=None, timeout=None, **popenkwargs)

This runs an external process using subprocess.Popen and gathers both the stdout and stderr output in an attribute for future use. stdin data can be piped to the process initially, by providing the data as a str or bytes during initialization.

After initializing a ProcessOutput object with a command to run, and optional stdin input data, it can either be used as a context manager or the run() method must be called.

timeout is passed to self.proc.wait() before returning the output.


  • args: Command arguments, same as subprocess.Popen.
  • stdin_data: str or bytes to send to command as stdin. Default: None
  • timeout: Time to wait for process after collecting data. Default: None
  • **popenkwargs: Any extra kwargs for Popen().


from outputcatcher import ProcessOutput

# Basic usage:
with ProcessOutput(['ls']) as p:

# Checking for stdout and stderr:
with ProcessOutput(['ls', '/totally_nonexistent_dir']) as p:
    if p.stdout:
        print('Wow, it really does exist: {}'.format(p.stdout.decode()))

# Without a context manager:
p = ProcessOutput(['ls'])
stdout, stderr =

# Sending stdin data to a process:
stdin_data = 'Hello cat!'
with ProcessOutput(['cat'], stdin_data=stdin_data) as p:
    assert p.stdout.decode() == stdin_data
    # cat received the data, and piped it back.

# Iterating over stdout data:
p = ProcessOutput(['ls'])
for line in p.iter_stdout():

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