[Patches] [ python-Patches-917095 ] dict type concat function (original) (raw)

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Tue Mar 16 14:35:00 EST 2004

Patches item #917095, was opened at 2004-03-16 00:33 Message generated for change (Comment added) made by tjreedy You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=305470&aid=917095&group_id=5470

Category: Core (C code) Group: Python 2.4 Status: Open Resolution: None Priority: 5 Submitted By: troy melhase (troy_melhase) Assigned to: Nobody/Anonymous (nobody) Summary: dict type concat function

Initial Comment: Adds a function to dictobject.c that allows python expressions like:

d = {2:3} d += {4:5} d {2: 3, 4: 5}

and like:

d = {2:3} e = d + {6:7} e {2: 3, 6: 7}

A few points:

Please advise, and thanks!

Comment By: Terry J. Reedy (tjreedy) Date: 2004-03-16 14:34

Message: Logged In: YES user_id=593130

I am usually big on orthogonality, but it never occurred to me that dicts should be addible, although sets are. What should be the result of {1:2} + {1:3}? If not something like {1:set (2,3)}, the operator is not commutative.

The reference to sets and | is that | is used, I believe, for set union (addition) (and & for intersection). So, if there were to be a dict union operator, the symbol should be | rather than +.

Comment By: troy melhase (troy_melhase) Date: 2004-03-16 14:22

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  1. adds new syntax but not new functionality

  2. writing d.update(x) is clearer and more flexible -- in Py2.4, update() can take the same arguments as dict()

The same is true of list.extend and list.append. After years of writing python code, I still forget that I can't add dictionaries to one another, which for me at least, violates the principle of least surprise.

The patch doesn't provide the same behavior as the update method; when adding two dicts, a third dict is returned. Only in the case of augmented assignment is the update method similar.

Moreover, every other basic container-ish type supports addition of the same type (or similar type). It seems to me that the dict type is missing a feature, not the other way around.

  1. concept doesn't usefully extend to - and *

I could see it extending to -, but you're right that it doesn't extend to *.

  1. possibly confusing given the | operator is used for sets

I don't understand this point, could you elaborate?

  1. {1:4}+{1:7} is not commutative and unlike list addition, there is no underlying order that makes the non- commutativity obvious. IOW, the operator may introduce a new class of hard to find bugs.

But unlike list addition, dict addition is commutative:

d = {2:3} e = {4:5} d + e == e + d True

Thanks for looking; could you advise again?

Comment By: Raymond Hettinger (rhettinger) Date: 2004-03-16 06:08

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I recommend against this for several reasons:

  1. adds new syntax but not new functionality

  2. concept doesn't usefully extend to - and *

  3. writing d.update(x) is clearer and more flexible -- in Py2.4, update() can take the same arguments as dict()

  4. no use cases showing the + operator to be more expressive

  5. possibly confusing given the | operator is used for sets

  6. {1:4}+{1:7} is not commutative and unlike list addition, there is no underlying order that makes the non- commutativity obvious. IOW, the operator may introduce a new class of hard to find bugs.

If for some reason this gets approved, please assign back to me for implementation, documentation, and unittests.

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