diff --git a/pep-0279.txt b/pep-0279.txt index 0204f813d..f90ce9dc9 100644 --- a/pep-0279.txt +++ b/pep-0279.txt @@ -7,7 +7,7 @@ Status: Draft Type: Standards Track Created: 30-Jan-2002 Python-Version: 2.3 -Post-History: +Post-History: Abstract @@ -45,7 +45,7 @@ Rationale 2. Provide a generator alternative to list comprehensions [3] making generator creation as convenient as list creation. - 3. Extend the syntax of the 'yield' keyword to enable two way + 3. Extend the syntax of the 'yield' keyword to enable generator parameter passing. The resulting increase in power simplifies the creation of consumer streams which have a complex execution state and/or variable state. @@ -57,7 +57,8 @@ Rationale All of the suggestions are designed to take advantage of the existing implementation and require little additional effort to incorporate. Each is backward compatible and requires no new - keywords. + keywords. These generator tools go into Python 2.3 when + generators become final and are not imported from __future__. Specification for new built-ins: @@ -149,33 +150,20 @@ Specification for new built-ins: Specification for Generator Comprehensions: If a list comprehension starts with a 'yield' keyword, then - express the remainder of the statement as generator. For example: + express the comprehension with a generator. For example: - g = [yield (len(line),line) for line in file.readline() if len(line)>5] - print g.next() - print g.next() + g = [yield (len(line),line) for line in file if len(line)>5] This would be implemented as if it had been written: - def __temp_gen(): - for line in file.readline(): - if len(line) > 5: - yield (len(line), line) - g = __temp_gen() - print g.next() - print g.next() + class __Temp: + def __iter__(self): + for line in file: + if len(line) > 5: + yield (len(line), line) + g = __Temp() - Note A: There is a difference in the above implementation as - compared to list comprehensions. For a generator comprehension, - the variables are created in a separate scope while list - comprehensions use the enclosing scope. If this PEP is accepted, - the parser should generate byte code that eliminates this - difference by passing the line variable in the enclosing scope and - using that same variable passed by reference inside the generator. - This will make the behavior of generator comprehension identical - to that of list comprehensions. - - Note B: There is some debate about whether the enclosing brackets + Note A: There is some debate about whether the enclosing brackets should be part of the syntax for generator comprehensions. On the plus side, it neatly parallels list comprehensions and would be immediately recognizable as a similar form with similar internal @@ -190,7 +178,7 @@ Specification for Generator Comprehensions: in fact helpful. -Specification for two-way Generator Parameter Passing: +Specification for Generator Parameter Passing: 1. Allow 'yield' to assign a value as in: @@ -209,7 +197,7 @@ Specification for two-way Generator Parameter Passing: The control flow is unchanged by this proposal. The only change is that a value can be sent into the generator. By analogy, consider the quality improvement from GOSUB (which had no argument - passing mechanism) to modern procedure calls (which pass in + passing mechanism) to modern procedure calls (which can pass in arguments and return values). Most of the underlying machinery is already in place, only the @@ -234,13 +222,13 @@ Specification for two-way Generator Parameter Passing: threading module with its attendant mutexes, semaphores, and data queues. A class-based approach competes well when there are no complex execution states or variable states. When the complexity - increases, generators with two-way communication are much simpler + increases, generators with parameter passing are much simpler because they automatically save state (unlike classes which must explicitly save the variable and execution state in instance variables). - Example of a Complex Consumer + Example of a Complex Consumer The encoder for arithmetic compression sends a series of fractional values to a complex, lazy consumer. That consumer @@ -281,6 +269,15 @@ Specification for two-way Generator Parameter Passing: more thumbnails on it. + Example of a Producer and Consumer Used Together in a Pipelike Fashion + + 'Analogy to: source | upper | sink' + sink = sinkgen() + sink.next() + for word in source(): + sink.next( word.upper() ) + + Specification for Generator Exception Passing: Add a .throw(exception) method to the resulting generator as in: @@ -311,22 +308,29 @@ Specification for Generator Exception Passing: putting the exception in another location. The word throw is already associated with exceptions in other languages. + Note B: The throw syntax should exactly match raise's syntax including: + raise string g.throw(string) + raise string, data g.throw(string,data) + raise class, instance g.throw(class,instance) + raise instance g.throw(instance) + raise g.throw() + References - + [1] PEP 255 Simple Generators http://python.sourceforge.net/peps/pep-0255.html [2] PEP 212 Loop Counter Iteration http://python.sourceforge.net/peps/pep-0212.html - [3] PEP 202 List Comprehensions + [3] PEP 202 List Comprehensions http://python.sourceforge.net/peps/pep-0202.html [4] There have been several discussion on comp.lang.python which helped tease out these proposals: - Indexed Function + Indexed Function http://groups.google.com/groups?hl=en&th=33f778d92dd5720a Xmap, Xfilter, Xzip and Two-way Generator Communication @@ -339,10 +343,10 @@ References http://groups.google.com/groups?hl=en&th=215e6e5a7bfd526&rnum=2 Discussion Draft of this PEP - http://groups.google.com/groups?hl=en&th=df8b5e7709957eb7 + http://groups.google.com/groups?hl=en&th=df8b5e7709957eb7 [5] Dr. David Mertz's draft column for Charming Python. - href="http://gnosis.cx/publish/programming/charming_python_b5.txt + http://gnosis.cx/publish/programming/charming_python_b5.txt Copyright @@ -356,3 +360,5 @@ mode: indented-text indent-tabs-mode: nil fill-column: 70 End: + +