Capture stdout from a script in Python

Question

suppose there is a script doing something like this:

# module writer.py
import sys

def write():
    sys.stdout.write("foobar")

Now suppose I want to capture the output of the write function and store it in a variable for further processing. The naive solution was:

# module mymodule.py
from writer import write

out = write()
print out.upper()

But this doesn't work. I come up with another solution and it works, but please, let me know if there is a better way to solve the problem. Thanks

import sys
from cStringIO import StringIO

# setup the environment
backup = sys.stdout

# ####
sys.stdout = StringIO()     # capture output
write()
out = sys.stdout.getvalue() # release output
# ####

sys.stdout.close()  # close the stream 
sys.stdout = backup # restore original stdout

print out.upper()   # post processing

Answer 1

Setting stdout is a reasonable way to do it. Another is to run it as another process:

import subprocess

proc = subprocess.Popen(["python", "-c", "import writer; writer.write()"], stdout=subprocess.PIPE)
out = proc.communicate()[0]
print out.upper()

Answer 2

Here is a context manager version of your code. It yields a list of two values; the first is stdout, the second is stderr.

import contextlib
@contextlib.contextmanager
def capture():
    import sys
    from cStringIO import StringIO
    oldout,olderr = sys.stdout, sys.stderr
    try:
        out=[StringIO(), StringIO()]
        sys.stdout,sys.stderr = out
        yield out
    finally:
        sys.stdout,sys.stderr = oldout, olderr
        out[0] = out[0].getvalue()
        out[1] = out[1].getvalue()

with capture() as out:
    print 'hi'

Answer 3

For future visitors: Python 3.4 contextlib provides for this directly (see Python contextlib help) via the redirect_stdout context manager:

from contextlib import redirect_stdout
import io

f = io.StringIO()
with redirect_stdout(f):
    help(pow)
s = f.getvalue()

Answer 4

Or maybe use functionality that is already there...

from IPython.utils.capture import capture_output

with capture_output() as c:
    print('some output')

c()

print c.stdout

Answer 5

This is the decorator counterpart of my original code.

writer.py remains the same:

import sys

def write():
    sys.stdout.write("foobar")

mymodule.py sligthly gets modified:

from writer import write as _write
from decorators import capture

@capture
def write():
    return _write()

out = write()
# out post processing...

And here is the decorator:

def capture(f):
    """
    Decorator to capture standard output
    """
    def captured(*args, **kwargs):
        import sys
        from cStringIO import StringIO

        # setup the environment
        backup = sys.stdout

        try:
            sys.stdout = StringIO()     # capture output
            f(*args, **kwargs)
            out = sys.stdout.getvalue() # release output
        finally:
            sys.stdout.close()  # close the stream 
            sys.stdout = backup # restore original stdout

        return out # captured output wrapped in a string

    return captured

Answer 6

Starting with Python 3 you can also use sys.stdout.buffer.write() to write (already) encoded byte strings to stdout (see stdout in Python 3). When you do that, the simple StringIO approach doesn't work because neither sys.stdout.encoding nor sys.stdout.buffer would be available.

Starting with Python 2.6 you can use the TextIOBase API, which includes the missing attributes:

import sys
from io import TextIOWrapper, BytesIO

# setup the environment
old_stdout = sys.stdout
sys.stdout = TextIOWrapper(BytesIO(), sys.stdout.encoding)

# do some writing (indirectly)
write("blub")

# get output
sys.stdout.seek(0)      # jump to the start
out = sys.stdout.read() # read output

# restore stdout
sys.stdout.close()
sys.stdout = old_stdout

# do stuff with the output
print(out.upper())

This solution works for Python 2 >= 2.6 and Python 3. Please note that our sys.stdout.write() only accepts unicode strings and sys.stdout.buffer.write() only accepts byte strings. This might not be the case for old code, but is often the case for code that is built to run on Python 2 and 3 without changes.

If you need to support code that sends byte strings to stdout directly without using stdout.buffer, you can use this variation:

class StdoutBuffer(TextIOWrapper):
    def write(self, string):
        try:
            return super(StdoutBuffer, self).write(string)
        except TypeError:
            # redirect encoded byte strings directly to buffer
            return super(StdoutBuffer, self).buffer.write(string)

You don't have to set the encoding of the buffer the sys.stdout.encoding, but this helps when using this method for testing/comparing script output.

Answer 7

The question here (the example of how to redirect output, not the tee part) uses os.dup2 to redirect a stream at the OS level. That is nice because it will apply to commands that you spawn from your program as well.

Answer 8

I think You should look at these four objects:

from test.test_support import captured_stdout, captured_output, \
    captured_stderr, captured_stdin

Example:

from writer import write

with captured_stdout() as stdout:
    write()
print stdout.getvalue().upper()

UPD: As Eric said in a comment, one shouldn't use they directly, so I copied and pasted it.

# Code from test.test_support:
import contextlib
import sys

@contextlib.contextmanager
def captured_output(stream_name):
    """Return a context manager used by captured_stdout and captured_stdin
    that temporarily replaces the sys stream *stream_name* with a StringIO."""
    import StringIO
    orig_stdout = getattr(sys, stream_name)
    setattr(sys, stream_name, StringIO.StringIO())
    try:
        yield getattr(sys, stream_name)
    finally:
        setattr(sys, stream_name, orig_stdout)

def captured_stdout():
    """Capture the output of sys.stdout:

       with captured_stdout() as s:
           print "hello"
       self.assertEqual(s.getvalue(), "hello")
    """
    return captured_output("stdout")

def captured_stderr():
    return captured_output("stderr")

def captured_stdin():
    return captured_output("stdin")

Answer 9

I like the contextmanager solution however if you need the buffer stored with the open file and fileno support you could do something like this.

import six
from six.moves import StringIO


class FileWriteStore(object):
    def __init__(self, file_):
        self.__file__ = file_
        self.__buff__ = StringIO()

    def __getattribute__(self, name):
        if name in {
            "write", "writelines", "get_file_value", "__file__",
                "__buff__"}:
            return super(FileWriteStore, self).__getattribute__(name)
        return self.__file__.__getattribute__(name)

    def write(self, text):
        if isinstance(text, six.string_types):
            try:
                self.__buff__.write(text)
            except:
                pass
        self.__file__.write(text)

    def writelines(self, lines):
        try:
            self.__buff__.writelines(lines)
        except:
            pass
        self.__file__.writelines(lines)

    def get_file_value(self):
        return self.__buff__.getvalue()

use

import sys
sys.stdout = FileWriteStore(sys.stdout)
print "test"
buffer = sys.stdout.get_file_value()
# you don't want to print the buffer while still storing
# else it will double in size every print
sys.stdout = sys.stdout.__file__
print buffer