[Python-Dev] unexpected import behaviour (original) (raw)

Michael Foord fuzzyman at voidspace.org.uk
Thu Jul 29 15:05:37 CEST 2010

On 29/07/2010 07:32, Daniel Waterworth wrote:

Hi,

I'm not sure if this is a bug or not, I certainly didn't expect it. If you create a file called test.py with the following contents,

The issue is that when your code is executed as a script it is run as the main module and not as the test module. When you import test you then import the same module with a different name - and your Test class is recreated (so main.Test is then different from test.Test). When you import your code as test and it then reimports itself it is only created once.

This is expected behaviour (not a bug), but it frequently confuses even relatively experienced programmers (it can happen by accident and cause hard to track down bugs) and I personally think that Python would be improved by issuing a warning if a main script reimports itself.

All the best,

Michael Foord

class Test: pass

def test1(): import test print Test == test.Test if name == 'main': test1() and then run it ($ python test.py), it'll print False. Now try: $python import test test.test1() and it'll print True. Is this behaviour expected? What was the rationale for it if is? Thanks, Daniel

-- http://www.ironpythoninaction.com/ http://www.voidspace.org.uk/blog

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