Why I can call 'print' from 'eval'

问题:

For code:

#!/usr/bin/python

src = """
print '!!!'
import os
"""

obj = compile(src, '', 'exec')
eval(obj, {'__builtins__': False})

I get output:

!!!
Traceback (most recent call last):
  File "./test.py", line 9, in <module>
    eval(obj, {'__builtins__': False})
  File "", line 3, in <module>

ImportError: __import__ not found

Both 'print' and 'import' are language construct. Why does 'eval' restrict using of 'import' but doesn't restrict 'print'?

P.S. I'm using python 2.6

UPDATE: Question is not "Why does import not work?" but "Why does print work?" Are there some architecture restrictions or something else?

回答1:

The __import__ method is invoked by the import keyword: python.org

If you want to be able to import a module you need to leave the __import__ method in the builtins:

src = """
print '!!!'
import os
"""

obj = compile(src, '', 'exec')
eval(obj, {'__builtins__': {'__import__':__builtins__.__import__}})

回答2:

In your eval the call to import is made successfully however import makes use of the __import__ method in builtins which you have made unavailable in your exec. This is the reason why you are seeing

ImportError: __import__ not found

print doesn't depend on any builtins so works OK.

You could pass just __import__ from builtins with something like:

eval(obj, {'__builtins__' : {'__import__' :__builtins__.__import__}})

回答3:

print works because you specified 'exec' to the compile function call.

回答4:

import calls the global/builtin __import__ function; if there isn't one to be found, import fails.

print does not rely on any globals to do its work. That is why print works in your example, even though you do not use the available __builtins__.