Raymond Hettinger's latest updates.
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Barry Warsaw
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pep-0279.txt
74
pep-0279.txt
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@ -7,7 +7,7 @@ Status: Draft
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Type: Standards Track
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Created: 30-Jan-2002
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Python-Version: 2.3
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Post-History:
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Post-History:
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Abstract
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@ -45,7 +45,7 @@ Rationale
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2. Provide a generator alternative to list comprehensions [3]
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making generator creation as convenient as list creation.
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3. Extend the syntax of the 'yield' keyword to enable two way
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3. Extend the syntax of the 'yield' keyword to enable generator
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parameter passing. The resulting increase in power simplifies
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the creation of consumer streams which have a complex execution
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state and/or variable state.
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@ -57,7 +57,8 @@ Rationale
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All of the suggestions are designed to take advantage of the
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existing implementation and require little additional effort to
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incorporate. Each is backward compatible and requires no new
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keywords.
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keywords. These generator tools go into Python 2.3 when
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generators become final and are not imported from __future__.
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Specification for new built-ins:
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@ -149,33 +150,20 @@ Specification for new built-ins:
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Specification for Generator Comprehensions:
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If a list comprehension starts with a 'yield' keyword, then
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express the remainder of the statement as generator. For example:
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express the comprehension with a generator. For example:
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g = [yield (len(line),line) for line in file.readline() if len(line)>5]
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print g.next()
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print g.next()
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g = [yield (len(line),line) for line in file if len(line)>5]
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This would be implemented as if it had been written:
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def __temp_gen():
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for line in file.readline():
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if len(line) > 5:
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yield (len(line), line)
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g = __temp_gen()
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print g.next()
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print g.next()
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class __Temp:
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def __iter__(self):
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for line in file:
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if len(line) > 5:
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yield (len(line), line)
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g = __Temp()
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Note A: There is a difference in the above implementation as
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compared to list comprehensions. For a generator comprehension,
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the variables are created in a separate scope while list
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comprehensions use the enclosing scope. If this PEP is accepted,
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the parser should generate byte code that eliminates this
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difference by passing the line variable in the enclosing scope and
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using that same variable passed by reference inside the generator.
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This will make the behavior of generator comprehension identical
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to that of list comprehensions.
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Note B: There is some debate about whether the enclosing brackets
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Note A: There is some debate about whether the enclosing brackets
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should be part of the syntax for generator comprehensions. On the
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plus side, it neatly parallels list comprehensions and would be
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immediately recognizable as a similar form with similar internal
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@ -190,7 +178,7 @@ Specification for Generator Comprehensions:
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in fact helpful.
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Specification for two-way Generator Parameter Passing:
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Specification for Generator Parameter Passing:
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1. Allow 'yield' to assign a value as in:
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@ -209,7 +197,7 @@ Specification for two-way Generator Parameter Passing:
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The control flow is unchanged by this proposal. The only change
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is that a value can be sent into the generator. By analogy,
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consider the quality improvement from GOSUB (which had no argument
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passing mechanism) to modern procedure calls (which pass in
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passing mechanism) to modern procedure calls (which can pass in
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arguments and return values).
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Most of the underlying machinery is already in place, only the
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@ -234,13 +222,13 @@ Specification for two-way Generator Parameter Passing:
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threading module with its attendant mutexes, semaphores, and data
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queues. A class-based approach competes well when there are no
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complex execution states or variable states. When the complexity
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increases, generators with two-way communication are much simpler
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increases, generators with parameter passing are much simpler
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because they automatically save state (unlike classes which must
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explicitly save the variable and execution state in instance
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variables).
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Example of a Complex Consumer
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Example of a Complex Consumer
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The encoder for arithmetic compression sends a series of
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fractional values to a complex, lazy consumer. That consumer
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@ -281,6 +269,15 @@ Specification for two-way Generator Parameter Passing:
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more thumbnails on it.
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Example of a Producer and Consumer Used Together in a Pipelike Fashion
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'Analogy to: source | upper | sink'
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sink = sinkgen()
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sink.next()
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for word in source():
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sink.next( word.upper() )
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Specification for Generator Exception Passing:
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Add a .throw(exception) method to the resulting generator as in:
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@ -311,22 +308,29 @@ Specification for Generator Exception Passing:
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putting the exception in another location. The word throw is
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already associated with exceptions in other languages.
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Note B: The throw syntax should exactly match raise's syntax including:
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raise string g.throw(string)
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raise string, data g.throw(string,data)
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raise class, instance g.throw(class,instance)
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raise instance g.throw(instance)
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raise g.throw()
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References
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[1] PEP 255 Simple Generators
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http://python.sourceforge.net/peps/pep-0255.html
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[2] PEP 212 Loop Counter Iteration
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http://python.sourceforge.net/peps/pep-0212.html
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[3] PEP 202 List Comprehensions
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[3] PEP 202 List Comprehensions
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http://python.sourceforge.net/peps/pep-0202.html
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[4] There have been several discussion on comp.lang.python which helped
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tease out these proposals:
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Indexed Function
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Indexed Function
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http://groups.google.com/groups?hl=en&th=33f778d92dd5720a
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Xmap, Xfilter, Xzip and Two-way Generator Communication
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@ -339,10 +343,10 @@ References
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http://groups.google.com/groups?hl=en&th=215e6e5a7bfd526&rnum=2
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Discussion Draft of this PEP
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http://groups.google.com/groups?hl=en&th=df8b5e7709957eb7
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http://groups.google.com/groups?hl=en&th=df8b5e7709957eb7
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[5] Dr. David Mertz's draft column for Charming Python.
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href="http://gnosis.cx/publish/programming/charming_python_b5.txt
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http://gnosis.cx/publish/programming/charming_python_b5.txt
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Copyright
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@ -356,3 +360,5 @@ mode: indented-text
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indent-tabs-mode: nil
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fill-column: 70
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End:
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