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Contents

• Motivation
• Major Objections
  • Dict Union Is Not Commutative
  • Dict Union Will Be Inefficient
  • Dict Union Is Lossy
  • Only One Way To Do It
  • More Than One Way To Do It
  • Dict Union Makes Code Harder To Understand
  • What About The Full set API?
  • What About Mapping And MutableMapping?
• Rejected Ideas
  • Rejected Semantics
  • Rejected Alternatives
    • Use A Method
    • Use a Function
• Examples

This PEP proposes adding merge (|) and update (|=) operatorsto the built-in dict class.

Note

After this PEP was accepted, the decision was made to alsoimplement the new operators for several other standard librarymappings.

The current ways to merge two dicts have several disadvantages:

dict.update

d1.update(d2) modifies d1 in-place.e = d1.copy(); e.update(d2) is not an expression and needs atemporary variable.

{**d1, **d2}

Dict unpacking looks ugly and is not easily discoverable. Few peoplewould be able to guess what it means the first time they see it, orthink of it as the 'obvious way' to merge two dicts.

As Guido said:

I'm sorry for PEP 448, but even if you know about **d insimpler contexts, if you were to ask a typical Python user howto combine two dicts into a new one, I doubt many people wouldthink of {**d1, **d2}. I know I myself had forgotten aboutit when this thread started!

{**d1, **d2} ignores the types of the mappings and always returnsa dict. type(d1)({**d1, **d2}) fails for dict subclassessuch as defaultdict that have an incompatible __init__ method.

collections.ChainMap

ChainMap is unfortunately poorly-known and doesn't qualify as'obvious'. It also resolves duplicate keys in the opposite order tothat expected ('first seen wins' instead of 'last seen wins'). Likedict unpacking, it is tricky to get it to honor the desired subclass.For the same reason, type(d1)(ChainMap(d2, d1)) fails for somesubclasses of dict.

Further, ChainMaps wrap their underlying dicts, so writes to theChainMap will modify the original dict:

dict(d1, **d2)

This 'neat trick' is not well-known, and only works when d2 isentirely string-keyed:

The new operators will have the same relationship to thedict.update method as the list concatenate (+) and extend(+=) operators have to list.extend. Note that this issomewhat different from the relationship that |/|= have withset.update; the authors have determined that allowing the in-placeoperator to accept a wider range of types (as list does) is a moreuseful design, and that restricting the types of the binary operator'soperands (again, as list does) will help avoid silent errorscaused by complicated implicit type casting on both sides.

Key conflicts will be resolved by keeping the rightmost value. Thismatches the existing behavior of similar dict operations, wherethe last seen value always wins:

All of the above follow the same rule. This PEP takes the positionthat this behavior is simple, obvious, usually the behavior we want,and should be the default behavior for dicts. This means that dictunion is not commutative; in general d | e != e | d.

Similarly, the iteration order of the key-value pairs in thedictionary will follow the same semantics as the examples above, witheach newly added key (and its value) being appended to the currentsequence.

Dict union will return a new dict consisting of the left operandmerged with the right operand, each of which must be a dict (or aninstance of a dict subclass). If a key appears in both operands,the last-seen value (i.e. that from the right-hand operand) wins:

The augmented assignment version operates in-place:

Augmented assignment behaves identically to the update methodcalled with a single positional argument, so it also accepts anythingimplementing the Mapping protocol (more specifically, anything withthe keys and __getitem__ methods) or iterables of key-valuepairs. This is analogous to list += and list.extend, whichaccept any iterable, not just lists. Continued from above:

When new keys are added, their order matches their order within theright-hand mapping, if any exists for its type.

One of the authors has written a C implementation.

An approximate pure-Python implementation is:

Dict Union Is Not Commutative

Union is commutative, but dict union will not be (d | e != e | d).

Response

There is precedent for non-commutative unions in Python:

While the results may be equal, they are distinctly different. Ingeneral, a | b is not the same operation as b | a.

Dict Union Will Be Inefficient

Giving a pipe operator to mappings is an invitation to writing codethat doesn't scale well. Repeated dict union is inefficient:d | e | f | g | h creates and destroys three temporary mappings.

{**d1, **d2} ignores the types of the mappings and always returnsa dict. type(d1)({**d1, **d2}) fails for dict subclassessuch as defaultdict that have an incompatible __init__ method.

collections.ChainMap

ChainMap is unfortunately poorly-known and doesn't qualify as'obvious'. It also resolves duplicate keys in the opposite order tothat expected ('first seen wins' instead of 'last seen wins'). Likedict unpacking, it is tricky to get it to honor the desired subclass.For the same reason, type(d1)(ChainMap(d2, d1)) fails for somesubclasses of dict.

Further, ChainMaps wrap their underlying dicts, so writes to theChainMap will modify the original dict:

dict(d1, **d2)

This 'neat trick' is not well-known, and only works when d2 isentirely string-keyed:

The new operators will have the same relationship to thedict.update method as the list concatenate (+) and extend(+=) operators have to list.extend. Note that this issomewhat different from the relationship that |/|= have withset.update; the authors have determined that allowing the in-placeoperator to accept a wider range of types (as list does) is a moreuseful design, and that restricting the types of the binary operator'soperands (again, as list does) will help avoid silent errorscaused by complicated implicit type casting on both sides.

Key conflicts will be resolved by keeping the rightmost value. Thismatches the existing behavior of similar dict operations, wherethe last seen value always wins:

All of the above follow the same rule. This PEP takes the positionthat this behavior is simple, obvious, usually the behavior we want,and should be the default behavior for dicts. This means that dictunion is not commutative; in general d | e != e | d.

Similarly, the iteration order of the key-value pairs in thedictionary will follow the same semantics as the examples above, witheach newly added key (and its value) being appended to the currentsequence.

Dict union will return a new dict consisting of the left operandmerged with the right operand, each of which must be a dict (or aninstance of a dict subclass). If a key appears in both operands,the last-seen value (i.e. that from the right-hand operand) wins:

The augmented assignment version operates in-place:

Augmented assignment behaves identically to the update methodcalled with a single positional argument, so it also accepts anythingimplementing the Mapping protocol (more specifically, anything withthe keys and __getitem__ methods) or iterables of key-valuepairs. This is analogous to list += and list.extend, whichaccept any iterable, not just lists. Continued from above:

When new keys are added, their order matches their order within theright-hand mapping, if any exists for its type.

One of the authors has written a C implementation.

An approximate pure-Python implementation is:

Dict Union Is Not Commutative

Union is commutative, but dict union will not be (d | e != e | d).

Response

There is precedent for non-commutative unions in Python:

While the results may be equal, they are distinctly different. Ingeneral, a | b is not the same operation as b | a.

Dict Union Will Be Inefficient

Giving a pipe operator to mappings is an invitation to writing codethat doesn't scale well. Repeated dict union is inefficient:d | e | f | g | h creates and destroys three temporary mappings.

Response

The same argument applies to sequence concatenation.

Sequence concatenation grows with the total number of items in thesequences, leading to O(N**2) (quadratic) performance. Dict union islikely to involve duplicate keys, so the temporary mappings willnot grow as fast.

Just as it is rare for people to concatenate large numbers of lists ortuples, the authors of this PEP believe that it will be rare forpeople to merge large numbers of dicts. collections.Counter is adict subclass that supports many operators, and there are no knownexamples of people having performance issues due to combining largenumbers of Counters. Further, a survey of the standard library by theauthors found no examples of merging more than two dicts, so this isunlikely to be a performance problem in practice.. 'Everything isfast for small enough N'.

If one expects to be merging a large number of dicts where performanceis an issue, it may be better to use an explicit loop and in-placemerging:

Dict Union Is Lossy

Dict union can lose data (values may disappear); no other form ofunion is lossy.

Response

It isn't clear why the first part of this argument is a problem.dict.update() may throw away values, but not keys; that isexpected behavior, and will remain expected behavior regardless ofwhether it is spelled as update() or |.

Other types of union are also lossy, in the sense of not beingreversible; you cannot get back the two operands given only the union.a | b 365.. what are a and b?

Only One Way To Do It

Dict union will violate the Only One Way koan from the Zen.

Response

There is no such koan. 'Only One Way' is a calumny about Pythonoriginating long ago from the Perl community.

More Than One Way To Do It

Okay, the Zen doesn't say that there should be Only One Way To Do It.But it does have a prohibition against allowing 'more than one way todo it'.

Response

There is no such prohibition. The 'Zen of Python' merely expresses apreference for 'only one obvious way':

The emphasis here is that there should be an obvious way to do 'it'.In the case of dict update operations, there are at least twodifferent operations that we might wish to do:

• Update a dict in place: The Obvious Way is to use the update()method. If this proposal is accepted, the |= augmentedassignment operator will also work, but that is a side-effect of howaugmented assignments are defined. Which you choose is a matter oftaste.
• Merge two existing dicts into a third, new dict: This PEP proposesthat the Obvious Way is to use the | merge operator.

In practice, this preference for 'only one way' is frequently violatedin Python. For example, every for loop could be re-written as awhile loop; every if block could be written as an if/else block. List, set and dict comprehensions could all bereplaced by generator expressions. Lists offer no fewer than fiveways to implement concatenation:

• Concatenation operator: a + b
• In-place concatenation operator: a += b
• Slice assignment: a[len(a):] = b
• Sequence unpacking: [*a, *b]
• Extend method: a.extend(b)

Whats taking up all the space on my hard drive. We should not be too strict about rejecting useful functionalitybecause it violates 'only one way'.

Dict Union Makes Code Harder To Understand

Dict union makes it harder to tell what code means. To paraphrase theobjection rather than quote anyone in specific: 'If I seespam | eggs, I can't tell what it does unless I know what spamand eggs are'.

Response

This is very true. But it is equally true today, where the use of the| operator could mean any of:

• int/bool bitwise-or
• set/frozenset union
• any other overloaded operation

Adding dict union to the set of possibilities doesn't seem to makeit harder to understand the code. No more work is required todetermine that spam and eggs are mappings than it would taketo determine that they are sets, or integers. And good namingconventions will help:

What About The Full set API?

dicts are 'set like', and should support the full collection of setoperators: |, &, ^, and -.

Response

This PEP does not take a position on whether dicts should support thefull collection of set operators, and would prefer to leave that for alater PEP (one of the authors is interested in drafting such a PEP).For the benefit of any later PEP, a brief summary follows.

Set symmetric difference (^) is obvious and natural. For example,given two dicts:

the symmetric difference d1 ^ d2 would be{'spam': 1, 'ham': 3}.

Set difference (-) is also obvious and natural, and an earlierversion of this PEP included it in the proposal. Given the dictsabove, we would have d1 - d2 be {'spam': 1} and d2 - d1 be{'ham': 3}.

Set intersection (&) is a bit more problematic. While it is easyto determine the intersection of keys in two dicts, it is not clearwhat to do with the values. Given the two dicts above, it isobvious that the only key of d1 & d2 must be 'eggs'. 'Lastseen wins', however, has the advantage of consistency with other dictoperations (and the proposed union operators).

What About Mapping And MutableMapping?

collections.abc.Mapping and collections.abc.MutableMappingshould define | and |=, so subclasses could just inherit thenew operators instead of having to define them.

Response

There are two primary reasons why adding the new operators to theseclasses would be problematic:

• Currently, neither defines a copy method, which would benecessary for | to create a new instance.
• Adding |= to MutableMapping (or a copy method toMapping) would create compatibility issues for virtualsubclasses.

Rejected Semantics

There were at least four other proposed solutions for handlingconflicting keys. These alternatives are left to subclasses of dict.

Raise

It isn't clear that this behavior has many use-cases or will be oftenuseful, but it will likely be annoying as any use of the dict unionoperator would have to be guarded with a try/except clause.

Add The Values (As Counter Does, with +)

Too specialised to be used as the default behavior.

Leftmost Value (First-Seen) Wins

It isn't clear that this behavior has many use-cases. In fact, onecan simply reverse the order of the arguments:

Concatenate Values In A List

This is likely to be too specialised to be the default. It is notclear what to do if the values are already lists:

Should this give {'a': [1, 2, 3, 4]} or{'a': [[1, 2], [3, 4]]}?

Rejected Alternatives

Use The Addition Operator

This PEP originally started life as a proposal for dict addition,using the + and += operator. That choice proved to beexceedingly controversial, with many people having serious objectionsto the choice of operator. For details, see previous versions of thePEP and the mailing list discussions.

Use The Left Shift Operator

The << operator didn't seem to get much support on Python-Ideas,but no major objections either. Perhaps the strongest objection wasChris Angelico's comment

The 'cuteness' value of abusing the operator to indicateinformation flow got old shortly after C++ did it.

Use A New Left Arrow Operator

Another suggestion was to create a new operator <-. Unfortunatelythis would be ambiguous, d <- e could mean d merge e ord less-than minus e.

Use A Method

A dict.merged() method would avoid the need for an operator atall. One subtlety is that it would likely need slightly differentimplementations when called as an unbound method versus as a boundmethod.

As an unbound method, the behavior could be similar to:

As a bound method, the behavior could be similar to:

Advantages

• Arguably, methods are more discoverable than operators.
• The method could accept any number of positional and keywordarguments, avoiding the inefficiency of creating temporary dicts.
• Accepts sequences of (key, value) pairs like the updatemethod.
• Being a method, it is easy to override in a subclass if you needalternative behaviors such as 'first wins', 'unique keys', etc.

Disadvantages

• Would likely require a new kind of method decorator which combinedthe behavior of regular instance methods and classmethod. Itwould need to be public (but not necessarily a builtin) for thoseneeding to override the method. There is aproof of concept.
• It isn't an operator. Guido discusses why operators are useful.For another viewpoint, see Nick Coghlan's blog post.

Use a Function

Instead of a method, use a new built-in function merged(). Onepossible implementation could be something like this:

An alternative might be to forgo the arbitrary keywords, and take asingle keyword parameter that specifies the behavior on collisions:

Advantages

• Most of the same advantages of the method solutions above.
• Doesn't require a subclass to implement alternative behavior oncollisions, just a function.

Disadvantages

• May not be important enough to be a builtin.
• Hard to override behavior if you need something like 'first wins',without losing the ability to process arbitrary keyword arguments.

The authors of this PEP did a survey of third party libraries fordictionary merging which might be candidates for dict union.

This is a cursory list based on a subset of whatever arbitrarythird-party packages happened to be installed on one of the authors'computers, and may not reflect the current state of any package. Alsonote that, while further (unrelated) refactoring may be possible, therewritten version only adds usage of the new operators for anapples-to-apples comparison. It also reduces the result to anexpression when it is efficient to do so.

IPython/zmq/kernelapp.py

Before:

After:

matplotlib/delaunay/triangulate.py

Before:

Rewrite as:

numpy/ma/core.py

Before: Play ios games on tv.

After:

pygments/lexer.py

Before:

After:

sphinx/domains/__init__.py

Before:

After:

sphinx/ext/inheritance_diagram.py

Before:

After:

sphinx/quickstart.py

Before:

After:

sympy/parsing/maxima.py

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After:

sympy/printing/ccode.py and sympy/printing/fcode.py

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Before:

After:

sympy/utilities/runtests.py

Before:

After:

The above examples show that sometimes the | operator leads to aclear increase in readability, reducing the number of lines of codeand improving clarity. However other examples using the |operator lead to long, complex single expressions, possibly well overthe PEP 8 maximum line length of 80 columns. As with any otherlanguage feature, the programmer should use their own judgement aboutwhether | improves their code.

Mailing list threads (this is by no means an exhaustive list):

Merging two dictionaries in an expression is a frequently requestedfeature. For example:

Occasionally people request alternative behavior for the merge:

..including one proposal to treat dicts as sets of keys.

Ian Lee's proto-PEP, anddiscussion in 2015. Furtherdiscussion took place on Python-Ideas.

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(Observant readers will notice that one of the authors of this PEP wasmore skeptical of the idea in 2015.)

Adding a full complement of operators to dicts.

Discussion on Y-Combinator.

In direct response to an earlier draft of this PEP, Serhiy Storchakaraised a ticket in the bug tracker to replace thecopy(); update() idiom with dict unpacking.

This document is placed in the public domain or under theCC0-1.0-Universal license, whichever is more permissive.