Python 3: Get Standard Output and Standard Error from subprocess.run()

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On Python 3.7 or higher, if we pass in capture_output=True to subprocess.run(), the CompletedProcess object returned by run() will contain the stdout (standard output) and stderr (standard error) output of the subprocess:

from subprocess import run

p = run( [ 'echo', 'hello' ], capture_output=True )

print( 'exit status:', p.returncode )
print( 'stdout:', p.stdout.decode() )
print( 'stderr:', p.stderr.decode() )

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from subprocess import run

p = run( [ 'echo', 'hello' ], capture_output=True )

print( 'exit status:', p.returncode )
print( 'stdout:', p.stdout.decode() )
print( 'stderr:', p.stderr.decode() )

p.stdout and p.stderr are bytes (binary data), so if we want to use them as UTF-8 strings, we have to first .decode() them.

Alternatively, on any Python 3 version that supports subprocess.run() (i.e., 3.5 and up), we can pass in subprocess.PIPE into the stdout and stderr options to capture output:

from subprocess import run, PIPE

p = run( [ 'echo', 'hello' ], stdout=PIPE, stderr=PIPE )

print( 'exit status:', p.returncode )
print( 'stdout:', p.stdout.decode() )
print( 'stderr:', p.stderr.decode() )

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from subprocess import run, PIPE

p = run( [ 'echo', 'hello' ], stdout=PIPE, stderr=PIPE )

print( 'exit status:', p.returncode )
print( 'stdout:', p.stdout.decode() )
print( 'stderr:', p.stderr.decode() )

Because subprocess.run() waits for the command to finish running before returning, we'll receive the entire contents of stdout and stderr as a whole only after the command has completed. If we need to stream output as it appears in real time, we can use Popen instead.

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