IT Ninja IT Ninja - 2 months ago 22
Python Question

capturing dis.dis results

Is there any way to get the output of

dis.dis()
without redirecting
sys.stdout
? I have tried:

out=str(dis.dis())


and

out=""""""
out+=str(dis.dis())


However I soon found out that it returns
None
. Is there any way to fix this?

Answer

Unfortunately, in Python versions before 3.4 the dis module uses print statements to stdout, so it won't return anything directly useful. Either you have to re-implement the dis, disassemble and disassemble_string functions, or you temporarily replace sys.stdout with an alternative to capture the output:

import sys
from cStringIO import StringIO

out = StringIO()
stdout = sys.stdout
sys.stdout = out
dis.dis()
sys.stdout = stdout
out = out.getvalue()

This is actually best done using a context manager:

import sys
from contextlib import contextmanager
from cStringIO import StringIO

@contextmanager
def captureStdOut(output):
    stdout = sys.stdout
    sys.stdout = output
    yield
    sys.stdout = stdout

out = StringIO()
with captureStdOut(out):
    dis.dis()
print out.getvalue()

That way you are guaranteed to have stdout restored even if something goes wrong with dis. A little demonstration:

>>> out = StringIO()
>>> with captureStdOut(out):
...     dis.dis(captureStdOut)
... 
>>> print out.getvalue()
 83           0 LOAD_GLOBAL              0 (GeneratorContextManager)
              3 LOAD_DEREF               0 (func)
              6 LOAD_FAST                0 (args)
              9 LOAD_FAST                1 (kwds)
             12 CALL_FUNCTION_VAR_KW     0
             15 CALL_FUNCTION            1
             18 RETURN_VALUE        

In Python 3.4 and up, the relevant functions take a file parameter to redirect output to:

from io import StringIO

with StringIO() as out:
    dis.dis(file=out)
    print(out.get_value())
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