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PEP 275 -- Switching on Multiple Values

PEP:275
Title:Switching on Multiple Values
Author:mal at lemburg.com (Marc-André Lemburg)
Status:Rejected
Type:Standards Track
Created:10-Nov-2001
Python-Version:2.6
Post-History:

Rejection Notice

A similar PEP for Python 3000, PEP 3103 [2], was already rejected, so this proposal has no chance of being accepted either.

Abstract

This PEP proposes strategies to enhance Python's performance with respect to handling switching on a single variable having one of multiple possible values.

Problem

Up to Python 2.5, the typical way of writing multi-value switches has been to use long switch constructs of the following type:

if x == 'first state':
    ...
elif x == 'second state':
    ...
elif x == 'third state':
    ...
elif x == 'fourth state':
    ...
else:
    # default handling
    ...

This works fine for short switch constructs, since the overhead of repeated loading of a local (the variable x in this case) and comparing it to some constant is low (it has a complexity of O(n) on average). However, when using such a construct to write a state machine such as is needed for writing parsers the number of possible states can easily reach 10 or more cases.

The current solution to this problem lies in using a dispatch table to find the case implementing method to execute depending on the value of the switch variable (this can be tuned to have a complexity of O(1) on average, e.g. by using perfect hash tables). This works well for state machines which require complex and lengthy processing in the different case methods. It does not perform well for ones which only process one or two instructions per case, e.g.

def handle_data(self, data):
    self.stack.append(data)

A nice example of this is the state machine implemented in pickle.py which is used to serialize Python objects. Other prominent cases include XML SAX parsers and Internet protocol handlers.

Proposed Solutions

This PEP proposes two different but not necessarily conflicting solutions:

  1. Adding an optimization to the Python compiler and VM which detects the above if-elif-else construct and generates special opcodes for it which use a read-only dictionary for storing jump offsets.
  2. Adding new syntax to Python which mimics the C style switch statement.

The first solution has the benefit of not relying on adding new keywords to the language, while the second looks cleaner. Both involve some run-time overhead to assure that the switching variable is immutable and hashable.

Both solutions use a dictionary lookup to find the right jump location, so they both share the same problem space in terms of requiring that both the switch variable and the constants need to be compatible to the dictionary implementation (hashable, comparable, a==b => hash(a)==hash(b)).

Solution 1: Optimizing if-elif-else

Implementation:

It should be possible for the compiler to detect an if-elif-else construct which has the following signature:

if x == 'first':...
elif x == 'second':...
else:...

i.e. the left hand side always references the same variable, the right hand side a hashable immutable builtin type. The right hand sides need not be all of the same type, but they should be comparable to the type of the left hand switch variable.

The compiler could then setup a read-only (perfect) hash table, store it in the constants and add an opcode SWITCH in front of the standard if-elif-else byte code stream which triggers the following run-time behaviour:

At runtime, SWITCH would check x for being one of the well-known immutable types (strings, unicode, numbers) and use the hash table for finding the right opcode snippet. If this condition is not met, the interpreter should revert to the standard if-elif-else processing by simply skipping the SWITCH opcode and procedding with the usual if-elif-else byte code stream.

Issues:

The new optimization should not change the current Python semantics (by reducing the number of __cmp__ calls and adding __hash__ calls in if-elif-else constructs which are affected by the optimiztation). To assure this, switching can only safely be implemented either if a "from __future__" style flag is used, or the switching variable is one of the builtin immutable types: int, float, string, unicode, etc. (not subtypes, since it's not clear whether these are still immutable or not)

To prevent post-modifications of the jump-table dictionary (which could be used to reach protected code), the jump-table will have to be a read-only type (e.g. a read-only dictionary).

The optimization should only be used for if-elif-else constructs which have a minimum number of n cases (where n is a number which has yet to be defined depending on performance tests).

Solution 2: Adding a switch statement to Python

New Syntax

switch EXPR:
    case CONSTANT:
        SUITE
    case CONSTANT:
        SUITE
    ...
    else:
        SUITE

(modulo indentation variations)

The "else" part is optional. If no else part is given and none of the defined cases matches, no action is taken and the switch statement is ignored. This is in line with the current if-behaviour. A user who wants to signal this situation using an exception can define an else-branch which then implements the intended action.

Note that the constants need not be all of the same type, but they should be comparable to the type of the switch variable.

Implementation

The compiler would have to compile this into byte code similar to this:

def whatis(x):
    switch(x):
        case 'one':
            print '1'
        case 'two':
            print '2'
        case 'three':
            print '3'
        else:
            print "D'oh!"

into (omitting POP_TOP's and SET_LINENO's):

   6  LOAD_FAST         0 (x)
   9  LOAD_CONST        1 (switch-table-1)
  12  SWITCH            26 (to 38)

  14  LOAD_CONST        2 ('1')
  17  PRINT_ITEM
  18  PRINT_NEWLINE
  19  JUMP 43

  22  LOAD_CONST        3 ('2')
  25  PRINT_ITEM
  26  PRINT_NEWLINE
  27  JUMP 43

  30  LOAD_CONST        4 ('3')
  33  PRINT_ITEM
  34  PRINT_NEWLINE
  35  JUMP 43

  38  LOAD_CONST        5 ("D'oh!")
  41  PRINT_ITEM
  42  PRINT_NEWLINE

>>43  LOAD_CONST        0 (None)
  46  RETURN_VALUE

Where the 'SWITCH' opcode would jump to 14, 22, 30 or 38 depending on 'x'.

Thomas Wouters has written a patch which demonstrates the above. You can download it from [1].

Issues

The switch statement should not implement fall-through behaviour (as does the switch statement in C). Each case defines a complete and independent suite; much like in a if-elif-else statement. This also enables using break in switch statements inside loops.

If the interpreter finds that the switch variable x is not hashable, it should raise a TypeError at run-time pointing out the problem.

There have been other proposals for the syntax which reuse existing keywords and avoid adding two new ones ("switch" and "case"). Others have argued that the keywords should use new terms to avoid confusion with the C keywords of the same name but slightly different semantics (e.g. fall-through without break). Some of the proposed variants:

case EXPR:
    of CONSTANT:
        SUITE
    of CONSTANT:
        SUITE
    else:
        SUITE

case EXPR:
    if CONSTANT:
         SUITE
    if CONSTANT:
        SUITE
    else:
        SUITE

when EXPR:
    in CONSTANT_TUPLE:
        SUITE
    in CONSTANT_TUPLE:
        SUITE
    ...
else:
     SUITE

The switch statement could be extended to allow multiple values for one section (e.g. case 'a', 'b', 'c': ...). Another proposed extension would allow ranges of values (e.g. case 10..14: ...). These should probably be post-poned, but already kept in mind when designing and implementing a first version.

Examples

The following examples all use a new syntax as proposed by solution 2. However, all of these examples would work with solution 1 as well.

switch EXPR:                   switch x:
    case CONSTANT:                 case "first":
        SUITE                          print x
    case CONSTANT:                 case "second":
        SUITE                          x = x**2
    ...                                print x
    else:                          else:
        SUITE                          print "whoops!"


case EXPR:                     case x:
    of CONSTANT:                   of "first":
        SUITE                          print x
    of CONSTANT:                   of "second":
        SUITE                          print x**2
    else:                          else:
        SUITE                          print "whoops!"


case EXPR:                     case state:
    if CONSTANT:                   if "first":
         SUITE                         state = "second"
    if CONSTANT:                   if "second":
        SUITE                          state = "third"
    else:                          else:
        SUITE                          state = "first"


when EXPR:                     when state:
    in CONSTANT_TUPLE:             in ("first", "second"):
        SUITE                          print state
    in CONSTANT_TUPLE:                 state = next_state(state)
        SUITE                      in ("seventh",):
    ...                                print "done"
else:                                  break    # out of loop!
     SUITE                     else:
                                   print "middle state"
                                   state = next_state(state)

Here's another nice application found by Jack Jansen (switching on argument types):

switch type(x).__name__:
    case 'int':
        SUITE
    case 'string':
        SUITE

Scope

XXX Explain "from __future__ import switch"

Credits

  • Martin von Löwis (issues with the optimization idea)
  • Thomas Wouters (switch statement + byte code compiler example)
  • Skip Montanaro (dispatching ideas, examples)
  • Donald Beaudry (switch syntax)
  • Greg Ewing (switch syntax)
  • Jack Jansen (type switching examples)
Source: https://github.com/python/peps/blob/master/pep-0275.txt