[Python-checkins] r51664 - in python/trunk: Doc/lib/libdecimal.tex Lib/decimal.py Lib/test/test_decimal.py

Thu Aug 31 14:00:44 CEST 2006

Author: nick.coghlan
Date: Thu Aug 31 14:00:43 2006
New Revision: 51664
Modified:
 python/trunk/Doc/lib/libdecimal.tex
 python/trunk/Lib/decimal.py
 python/trunk/Lib/test/test_decimal.py
Log:
Fix the wrongheaded implementation of context management in the decimal module and add unit tests. (python-dev discussion is ongoing regarding what we do about Python 2.5)
Modified: python/trunk/Doc/lib/libdecimal.tex
==============================================================================

--- python/trunk/Doc/lib/libdecimal.tex	(original)
+++ python/trunk/Doc/lib/libdecimal.tex	Thu Aug 31 14:00:43 2006
@@ -435,26 +435,34 @@
 the \function{getcontext()} and \function{setcontext()} functions:
 
 \begin{funcdesc}{getcontext}{}
- Return the current context for the active thread. 
+ Return the current context for the active thread.
 \end{funcdesc} 
 
 \begin{funcdesc}{setcontext}{c}
- Set the current context for the active thread to \var{c}. 
+ Set the current context for the active thread to \var{c}.
 \end{funcdesc} 
 
 Beginning with Python 2.5, you can also use the \keyword{with} statement
-to temporarily change the active context. For example the following code
-increases the current decimal precision by 2 places, performs a
-calculation, and then automatically restores the previous context:
+to temporarily change the active context.
 
-\begin{verbatim}
-from __future__ import with_statement
-import decimal
-
-with decimal.getcontext() as ctx:
- ctx.prec += 2 # add 2 more digits of precision
- calculate_something()
+\begin{funcdesc}{localcontext}{\optional{c}}
+ Return a context manager that will set the current context for
+ the active thread to a copy of \var{c} on entry to the with statement
+ and restore the previous context when exiting the with statement.
+
+ For example the following code increases the current decimal precision
+ by 2 places, performs a calculation, and then automatically restores
+ the previous context:
+\begin{verbatim}
+ from __future__ import with_statement
+ import decimal
+
+ with decimal.localcontext() as ctx:
+ ctx.prec += 2 # add 2 more digits of precision
+ s = calculate_something()
+ s = +s # Round the final result back to the default precision
 \end{verbatim}
+\end{funcdesc}
 
 The context that's active in the body of the \keyword{with} statement is
 a \emph{copy} of the context you provided to the \keyword{with}
Modified: python/trunk/Lib/decimal.py
==============================================================================
--- python/trunk/Lib/decimal.py	(original)
+++ python/trunk/Lib/decimal.py	Thu Aug 31 14:00:43 2006
@@ -130,8 +130,11 @@
 'ROUND_DOWN', 'ROUND_HALF_UP', 'ROUND_HALF_EVEN', 'ROUND_CEILING',
 'ROUND_FLOOR', 'ROUND_UP', 'ROUND_HALF_DOWN',
 
+ # helper for context management
+ 'ContextManager',
+
 # Functions for manipulating contexts
- 'setcontext', 'getcontext'
+ 'setcontext', 'getcontext', 'localcontext'
 ]
 
 import copy as _copy
@@ -458,6 +461,49 @@
 
 del threading, local # Don't contaminate the namespace
 
+def localcontext(ctx=None):
+ """Return a context manager for a copy of the supplied context
+
+ Uses a copy of the current context if no context is specified
+ The returned context manager creates a local decimal context
+ in a with statement:
+ def sin(x):
+ with localcontext() as ctx:
+ ctx.prec += 2
+ # Rest of sin calculation algorithm
+ # uses a precision 2 greater than normal
+ return +s # Convert result to normal precision
+
+ def sin(x):
+ with localcontext(ExtendedContext):
+ # Rest of sin calculation algorithm
+ # uses the Extended Context from the
+ # General Decimal Arithmetic Specification
+ return +s # Convert result to normal context
+
+ """
+ # The below can't be included in the docstring until Python 2.6
+ # as the doctest module doesn't understand __future__ statements
+ """
+ >>> from __future__ import with_statement
+ >>> print getcontext().prec
+ 28
+ >>> with localcontext():
+ ... ctx = getcontext()
+ ... ctx.prec() += 2
+ ... print ctx.prec
+ ...
+ 30
+ >>> with localcontext(ExtendedContext):
+ ... print getcontext().prec
+ ...
+ 9
+ >>> print getcontext().prec
+ 28
+ """
+ if ctx is None: ctx = getcontext().copy()
+ return ContextManager(ctx.copy())
+
 
 ##### Decimal class ###########################################
 
@@ -2174,20 +2220,27 @@
 del name, val, globalname, rounding_functions
 
 class ContextManager(object):
- """Helper class to simplify Context management.
+ """Context manager class to support localcontext().
 
- Sample usage:
-
- with decimal.ExtendedContext:
- s = ...
- return +s # Convert result to normal precision
-
- with decimal.getcontext() as ctx:
- ctx.prec += 2
- s = ...
- return +s
+ Sets the supplied context in __enter__() and restores
+ the previous decimal context in __exit__()
 
 """
+ # The below can't be included in the docstring until Python 2.6
+ # as the doctest module doesn't understand __future__ statements
+ """
+ Sample usage:
+ >>> from __future__ import with_statement
+ >>> print getcontext().prec
+ 28
+ >>> ctx = Context(prec=15)
+ >>> with ContextManager(ctx):
+ ... print getcontext().prec
+ ...
+ 15
+ >>> print getcontext().prec
+ 28
+ """
 def __init__(self, new_context):
 self.new_context = new_context
 def __enter__(self):
@@ -2248,9 +2301,6 @@
 s.append('traps=[' + ', '.join([t.__name__ for t, v in self.traps.items() if v]) + ']')
 return ', '.join(s) + ')'
 
- def get_manager(self):
- return ContextManager(self.copy())
-
 def clear_flags(self):
 """Reset all flags to zero"""
 for flag in self.flags:
Modified: python/trunk/Lib/test/test_decimal.py
==============================================================================
--- python/trunk/Lib/test/test_decimal.py	(original)
+++ python/trunk/Lib/test/test_decimal.py	Thu Aug 31 14:00:43 2006
@@ -23,6 +23,7 @@
 you're working through IDLE, you can import this test module and call test_main()
 with the corresponding argument.
 """
+from __future__ import with_statement
 
 import unittest
 import glob
@@ -1064,6 +1065,43 @@
 self.assertNotEqual(id(c.flags), id(d.flags))
 self.assertNotEqual(id(c.traps), id(d.traps))
 
+class WithStatementTest(unittest.TestCase):
+ # Can't do these as docstrings until Python 2.6
+ # as doctest can't handle __future__ statements
+ def test_ContextManager(self):
+ # The basic context manager uses the supplied context
+ # without making a copy of it
+ orig_ctx = getcontext()
+ new_ctx = Context()
+ with ContextManager(new_ctx) as enter_ctx:
+ set_ctx = getcontext()
+ final_ctx = getcontext()
+ self.assert_(orig_ctx is final_ctx, 'did not restore context correctly')
+ self.assert_(new_ctx is set_ctx, 'did not set correct context')
+ self.assert_(set_ctx is enter_ctx, '__enter__ returned wrong context')
+
+ def test_localcontext(self):
+ # The helper function makes a copy of the supplied context
+ orig_ctx = getcontext()
+ with localcontext() as enter_ctx:
+ set_ctx = getcontext()
+ final_ctx = getcontext()
+ self.assert_(orig_ctx is final_ctx, 'did not restore context correctly')
+ self.assert_(orig_ctx is not set_ctx, 'did not copy the context')
+ self.assert_(set_ctx is enter_ctx, '__enter__ returned wrong context')
+
+ def test_localcontextarg(self):
+ # The helper function makes a copy of the supplied context
+ orig_ctx = getcontext()
+ new_ctx = Context(prec=42)
+ with localcontext(new_ctx) as enter_ctx:
+ set_ctx = getcontext()
+ final_ctx = getcontext()
+ self.assert_(orig_ctx is final_ctx, 'did not restore context correctly')
+ self.assert_(set_ctx.prec == new_ctx.prec, 'did not set correct context')
+ self.assert_(new_ctx is not set_ctx, 'did not copy the context')
+ self.assert_(set_ctx is enter_ctx, '__enter__ returned wrong context')
+
 def test_main(arith=False, verbose=None):
 """ Execute the tests.
 
@@ -1084,6 +1122,7 @@
 DecimalPythonAPItests,
 ContextAPItests,
 DecimalTest,
+ WithStatementTest,
 ]
 
 try:
