[Python-Dev] More data points (original) (raw)

Tim Peters tim.peters at gmail.com
Sun Sep 26 07:50:50 CEST 2004

[ncoghlan at iinet.net.au]

Returning to Tim's original infinite loop, the behaviour is interestingly variable.

List and array go into the infinite loop.

What happens when you mutate a list while iterating over it is defined, and an infinite loop is expected for that. Ditto for array.

Deque and dictionary both detect that the loop variable has been mutated and throw a specific exception.

That's because they never suffered from list's ill-advised documentation effectively blessing mutation while iterating <0.5 wink>.

Set throws the same exception as dictionary does (presumably, the main container inside 'set' is a dictionary)

Details of behaviour:

The last one is extremely surprising:

Python 2.4a3 (#16, Sep 21 2004, 17:33:57) [GCC 3.4.1 20040702 (Red Hat Linux 3.4.1-2)] on linux2 Type "help", "copyright", "credits" or "license" for more information.

...

>>> x {1: None, -1: None} >>> x.fromkeys(-y for y in x) {-1: None}

Are you sure get that? I get this:

x {1: None, -1: None} x.fromkeys(-y for y in x) {1: None, -1: None}

"x.fromkeys()" doesn't have anything to do with x. Any dict works same there:

{}.fromkeys(-y for y in x) {1: None, -1: None} {'a': 'b', 'c': 'd', 'e': 'f'}.fromkeys(-y for y in x) {1: None, -1: None}

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