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AllenDowney / ThinkPython2

LaTeX source and supporting code for Think Python, 2nd edition, by Allen Downey.
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Dict is now ordered. Maybe the book should be updated? #95

Open gbnewb opened 2 years ago

gbnewb commented 2 years ago

As of Python 3.6, for the CPython implementation of Python, dictionaries remember the order of items inserted. This is considered an implementation detail in Python 3.6; you need to use OrderedDict if you want insertion ordering that's guaranteed across other implementations of Python.

As of Python 3.7, this is a guaranteed language feature, not merely an implementation detail.

mcmwebsol commented 6 months ago

I think Section 11.1 should be updated regarding this.

AllenDowney commented 6 months ago

As I understand it, ordered dictionaries are not part of the Python language specification, they are an implementation detail of some versions of CPython.

The book specifies Python 3 but not which implementation or version. So I don't think I can generally say that dictionaries are ordered.

On Thu, Feb 22, 2024, at 11:26 AM, Joseph McMurry wrote:

I think Section 11.1 should be updated regarding this.

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mcmwebsol commented 6 months ago

As of Python 3.7 it's part of the Python language specification. https://docs.python.org/3/whatsnew/3.7.html - "the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec."

Also from https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects - "Dictionaries preserve insertion order. Note that updating a key does not affect the order. Keys added after deletion are inserted at the end."

AllenDowney commented 6 months ago

Fair enough -- but still, the book doesn't specify a version, and for the target audience of Think Python, it's a detail we don't need at this point.

On Thu, Feb 22, 2024, at 11:40 AM, Joseph McMurry wrote:

As of Python 3.7 it's part of the Python language specification. https://docs.python.org/3/whatsnew/3.7.html - "the insertion-order preservation nature of dict https://docs.python.org/3/library/stdtypes.html#typesmapping objects has been declared https://mail.python.org/pipermail/python-dev/2017-December/151283.html to be an official part of the Python language spec."

Also from https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects - "Dictionaries preserve insertion order. Note that updating a key does not affect the order. Keys added after deletion are inserted at the end."

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