External library uses print instead of return

Question

I have a problem with an external library that I use in my script.

I execute a function from this library, but the function prints the output directly. However, I need the output to check if there is a specific string in it.

How can I ensure that the output of the function in the external library comes in a variable so that I can make the comparisons with the string?

How tied are you to that external library? It sounds pretty badly designed. Are there any ways you can access the value more directly? — jonrsharpe, Nov 30 '18 at 17:05
This answer helped me with a similar issue in the past: https://stackoverflow.com/a/16571630/6321559 — it's-yer-boy-chet, Nov 30 '18 at 17:11

score 2 · Answer 1 · answered Nov 30 '18 at 17:11

If you really have no other choice, you could redirect stdout when you call the library. Here's a piece of code adapted from this answer :

def some_library():
    print("Should probably return instead of print.")

import sys
from io import StringIO

class redirected_stdout:
    def __init__(self):
        self._stdout = None
        self._string_io = None

    def __enter__(self):
        self._stdout = sys.stdout
        sys.stdout = self._string_io = StringIO()
        return self

    def __exit__(self, type, value, traceback):
        sys.stdout = self._stdout

    @property
    def string(self):
        return self._string_io.getvalue()

with redirected_stdout() as out:
    some_library()
    result = out.string

print("return" in result)
# True

It would be much cleaner to modify the library, though.

score 1 · Accepted Answer · answered Nov 30 '18 at 18:05

You can exchange sys.stdout with your buffer temporarily, and then check the buffer.

def external_method():
  print ("print something out, don't return")

class MyBuffer(object):
  def __init__(self):
    self.buffer = []
  def write(self, *args, **kwargs):
    self.buffer.append(args)

import sys
old_stdout = sys.stdout
sys.stdout = MyBuffer()
external_method()
my_buffer, sys.stdout = sys.stdout, old_stdout
print (my_buffer.buffer)

External library uses print instead of return

2 Answers2