[Python-Dev] Split switch statement (original) (raw)

Eric Sumner kd5bjo at gmail.com
Tue Jun 27 22:36:43 CEST 2006

One of the big problems here seems to be that an optimized switch statement requires some code to be evaluated fewer times than the rest of the switch statement, and there isn't any good way to make that happen with a single statement. Thus, I propose two statements: a 'dispatcher' statement and a 'switch' statement. The dispatcher statement defines the available cases, and generates a dispatcher object, and the switch statement specifies code to be run for each case.

#Sample 1: Primary dispatcher syntax dispatcher myEvents on e: on e: case None: Idle on getattr(e, 'type', None): from globalEvents: * case 42: myEvent # or whatever the user event code is #EOF

A dispatcher statement contains a sequence of 'on' blocks. Each 'on' block specifies an expression and a set of cases. The expression is stored as a lambda inside the dispatcher which is applied whenever the switch is run. Inside a 'on' block, there are two kinds of statements. 'case' evaluates its expression immediately, and associates it with a label; 'from' imports tests and labels from another dispatcher. If the result of any case expression is unhashable, an exception is raised.

#Sample 2: Shorthand dispatcher syntax dispatcher globalEvents: case pygame.KEYDOWN: KEYDOWN case pygame.KEYUP: KEYUP ... #EOF

Because dispatching on the switched value directly is so common, any 'from' or 'case' statements outside an 'on' block are considered to be applied to be inside an "on " block. The name for the switched value can be omitted if it's not needed.

#Sample 3: Using a dispatcher while True: ... switch events on pygame.event.poll(): case KEYUP, KEYDOWN: ... case myEvent: ... case Idle: ... else: ... #EOF

Internally, each switch statement has some unique identifier. Each dispatcher object maintains a list of the switch statements it has previously serviced. If this switch statement is new to this dispatcher, the dispatcher verifies that it might generate all of the cases that are specified in the switch. Otherwise, an exception is raised.

If the test passed (or was skipped due to previous experience), each of the 'on' expressions in the dispatcher is executed (in order) and their results are checked against the stored values. If no case (from the switch, not the dispatcher) matches, the switch's 'else' block is executed, if present. If more than one case (from the switch) matches, an exception is raised. Otherwise, the code from associated case block is executed.

-- Eric Sumner

PS. Yes, I know that's not how pygame handles idle events; it makes a better sample this way.

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