[Python-checkins] peps: some content to start things off (maybe someone else will add to it...)

[georg.brandl]

http://hg.python.org/peps/rev/bcf483d2b43d
changeset:   44:bcf483d2b43d
user:        Skip Montanaro <skip at pobox.com>
date:        Tue Jul 25 15:07:28 2000 +0000
summary:
  some content to start things off (maybe someone else will add to it...)

files:
  pep-0202.txt |  108 +++++++++++++++++++++++++++++++++++++++
  1 files changed, 108 insertions(+), 0 deletions(-)


diff --git a/pep-0202.txt b/pep-0202.txt
--- a/pep-0202.txt
+++ b/pep-0202.txt
@@ -6,6 +6,114 @@
 Status: Incomplete
 
 
+Introduction
+
+    This PEP describes a proposed syntactical extension to Python, list
+    comprehensions. 
+
+
+
+The Proposed Solution
+
+    It is proposed to allow conditional construction of list literals using
+    for and if clauses.  They would nest in the same way for loops and if
+    statements nest now.
+    
+
+
+Rationale
+
+    List comprehensions provide a more concise way to create lists in
+    situations where map() and filter() and/or nested loops would currently
+    be used.
+
+
+Examples
+
+    >>> print [i for i in range(10)]
+    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+
+    >>> print [i for i in range(20) if i%2 == 0]
+    [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
+
+    >>> nums = [1,2,3,4]
+    >>> fruit = ["Apples", "Peaches", "Pears", "Bananas"]
+    >>> print [i,f for i in nums for f in fruit]
+    [(1, 'Apples'), (1, 'Peaches'), (1, 'Pears'), (1, 'Bananas'),
+     (2, 'Apples'), (2, 'Peaches'), (2, 'Pears'), (2, 'Bananas'),
+     (3, 'Apples'), (3, 'Peaches'), (3, 'Pears'), (3, 'Bananas'),
+     (4, 'Apples'), (4, 'Peaches'), (4, 'Pears'), (4, 'Bananas')]
+    >>> print [i,f for i in nums for f in fruit if f[0] == "P"]
+    [(1, 'Peaches'), (1, 'Pears'),
+     (2, 'Peaches'), (2, 'Pears'),
+     (3, 'Peaches'), (3, 'Pears'),
+     (4, 'Peaches'), (4, 'Pears')]
+    >>> print [i,f for i in nums for f in fruit if f[0] == "P" if i%2 == 1]
+    [(1, 'Peaches'), (1, 'Pears'), (3, 'Peaches'), (3, 'Pears')]
+    >>> def zip(*args):
+    ...    return apply(map, (None,)+args)
+    ... 
+    >>> print [i for i in zip(nums,fruit) if i[0]%2==0]
+    [(2, 'Peaches'), (4, 'Bananas')]
+
+
+Reference Implementation
+
+    Please refer to 
+
+        https://sourceforge.net/patch/?func=detailpatch&patch_id=100654&group_id=5470
+
+    for a patch that adds list comprehensions to Python.
+
+
+
+Open Issues
+
+    Syntax
+
+        Several people proposed connecting or separating syntax between the
+        various clauses, for example, requiring a semicolon between them to
+        set them apart:
+
+            [i,f; for i in nums; for f in fruit; if f[0]=="P"; if i%2==1]
+
+        To minimize strain on the Python parser, Guido has suggested
+        requiring parentheses around the initial tuple:
+
+            [(i,f) for i in nums for f in fruit if f[0]=="P" if i%2==1]
+
+    Semantics
+
+        The semantics of multiple for clauses is not obvious to many
+        people.  Currently, it nests, so that
+
+            [i,f for i in nums for f in fruit]
+
+        is functionally equivalent to
+
+            tmp = []
+            for i in nums:
+                for f in fruit:
+                    tmp.append((i,f))
+
+        Other people would read it as if it executed
+
+            map(None, nums, fruit)
+
+        It's not clear that this is necessary.  The newly proposed zip()
+        builtin takes care of that case.
+
+    Stability of the Implementation
+
+        The current reference implementation is simply an adaptation of Greg
+        Ewing's original demonstration of the concept.  Other than tracking
+        changes to the source code to keep it a valid patch, reindenting the
+        code and switching to function prototypes, nothing has been done to
+        it.  This obviously raises some questions about how stable the code
+        is.  It has not had a lot of exercise, though the patch does include
+        a few test cases.
+
+
 Local Variables:
 mode: indented-text
 indent-tabs-mode: nil

-- 
Repository URL: http://hg.python.org/peps