why importing a module clashes with overridden print?

Question

I want to automatically add a timestamp to my print statements (in Python 2.x) by overriding it. It works, except when I import a module which also contains the override.

My main file testa.py:

from __future__ import print_function
import datetime
import testb

def print(*args, **kwargs):
    __builtins__.print(datetime.datetime.now(), ' ', end='')
    return __builtins__.print(*args, **kwargs)

print("from main file")
testb.MyClass()

The helper file testb.py

from __future__ import print_function
import datetime

def print(*args, **kwargs):
    __builtins__.print(datetime.datetime.now(), ' ', end='')
    return __builtins__.print(*args, **kwargs)

class MyClass():
    def __init__(self):
        print("from helper file")

When running testa:

2014-07-29 09:43:54.375000  from main file
Traceback (most recent call last):
  File "C:/testa.py", line 10, in <module>
    testb.MyClass()
  File "C:\testb.py", line 15, in __init__
    print("from helper file")
  File "C:\testb.py", line 10, in print
    __builtins__.print(datetime.datetime.now(), ' ', end='')
AttributeError: 'dict' object has no attribute 'print'

I think that when importing testb, its print definition somehow clashes with the one in testa but this is where my thinking reaches its limits.

score 4 · Accepted Answer · answered Jul 29 '14 at 08:00

Do not rely on the __builtins__. Instead, import __builtin__ and use __builtin__.print. See here: https://docs.python.org/2/reference/executionmodel.html

By default, when in the __main__ module, __builtins__ is the built-in module __builtin__ (note: no ‘s’); when in any other module, __builtins__ is an alias for the dictionary of the __builtin__ module itself.

CPython implementation detail: Users should not touch __builtins__; it is strictly an implementation detail. Users wanting to override values in the builtins namespace should import the __builtin__ (no ‘s’) module and modify its attributes appropriately.

score 1 · Answer 2 · answered Jul 29 '14 at 07:57

1

There might be a way to make this work but I suggest to look at the logging module instead.

print() is a nice little helper for you basic "output text" needs. Logging is much more powerful, it can add time stamps and "where was this printed" information to the output.

answered Jul 29 '14 at 07:57

Aaron Digulla

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I know the logging module and use it as well. The specific situation I face with my program (which is in debugging phase) drove me into overriding `print`. – WoJ Jul 29 '14 at 08:21

score 0 · Answer 3 · answered Jul 29 '14 at 08:06

Your code doesn't look too bad (though the DRY principle is being violated). It appears you have made an assignment (of a dict value) to the name builtins, which in my 2.7 installation is a module not a dict. But you haven't.

airhead:~ sholden$ python
Python 2.7.6 (default, Nov 19 2013, 03:12:29)
[GCC 4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.28)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> __builtins__
<module '__builtin__' (built-in)>
>>>

@rczajka give you the correct answer, which I've upvoted.

why importing a module clashes with overridden print?

3 Answers3