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Diffstat (limited to 'Doc/tut')
| -rw-r--r-- | Doc/tut/tut.tex | 84 |
1 files changed, 70 insertions, 14 deletions
diff --git a/Doc/tut/tut.tex b/Doc/tut/tut.tex index b00eaea..0dcac57 100644 --- a/Doc/tut/tut.tex +++ b/Doc/tut/tut.tex @@ -4338,7 +4338,7 @@ which returns an object with a \method{next()} method. If the class defines \method{next()}, then \method{__iter__()} can just return \code{self}: \begin{verbatim} ->>> class Reverse: +class Reverse: "Iterator for looping over a sequence backwards" def __init__(self, data): self.data = data @@ -4352,8 +4352,8 @@ which returns an object with a \method{next()} method. If the class defines return self.data[self.index] >>> for char in Reverse('spam'): - print char - +... print char +... m a p @@ -4371,13 +4371,13 @@ which statement was last executed). An example shows that generators can be trivially easy to create: \begin{verbatim} ->>> def reverse(data): - for index in range(len(data)-1, -1, -1): - yield data[index] - +def reverse(data): + for index in range(len(data)-1, -1, -1): + yield data[index] + >>> for char in reverse('golf'): - print char - +... print char +... f l o @@ -4894,7 +4894,7 @@ tasks in background while the main program continues to run: print 'The main program continues to run' \end{verbatim} -The principal challenge of multi-thread applications is coordinating +The principal challenge of multi-threaded applications is coordinating threads that share data or other resources. To that end, the threading module provides a number of synchronization primitives including locks, events, condition variables, and semaphores. @@ -4935,7 +4935,7 @@ This produces the following output: By default, informational and debugging messages are suppressed and the output is sent to standard error. Other output options include routing messages through email, datagrams, sockets, or to an HTTP Server. New -filters select different routing based on message priority: DEBUG, +filters can select different routing based on message priority: DEBUG, INFO, WARNING, ERROR, and CRITICAL. The logging system can be configured directly from Python or can be @@ -4967,12 +4967,12 @@ applications include caching objects that are expensive to create: ... def __repr__(self): ... return str(self.value) ... - >>> a = A(10) # create a reference + >>> a = A(10) # create a reference >>> d = weakref.WeakValueDictionary() >>> d['primary'] = a # does not create a reference - >>> d['primary'] # fetch the object if it is still alive + >>> d['primary'] # fetch the object if it is still alive 10 - >>> del a # remove the one reference + >>> del a # remove the one reference >>> gc.collect() # run garbage collection right away 0 >>> d['primary'] # entry was automatically removed @@ -5056,6 +5056,62 @@ want to run a full list sort: \end{verbatim} +\section{Tools for Working with Decimal Floating Point\label{decimal-fp}} + +The \module{decimal} module, offers a \class{Decimal} data type for +decimal floating point arithmetic. Compared to the built-in \class{float} +type implemented with binary floating point, the new class is especially +useful for financial applications and other uses which require exact +decimal representation, control over precision, control over rounding +to meet legal or regulatory requirements, tracking of significant +decimal places, or for applications where the user expects the results +to match hand calculations done as taught in school. + +For example, calculating a 5% tax on a 70 cent phone charge gives +different results in decimal floating point and binary floating point +with the difference being significant when rounding to the nearest +cent: + +\begin{verbatim} +>>> from decimal import * +>>> Decimal('0.70') * Decimal('1.05') +Decimal("0.7350") +>>> .70 * 1.05 +0.73499999999999999 +\end{verbatim} + +Note that the \class{Decimal} result keeps a trailing zero, automatically +inferring four place significance from two digit mulitiplicands. Decimal +reproduces mathematics as done by hand and avoids issues that can arise +when binary floating point cannot exactly represent decimal quantities. + +Exact representation enables the \class{Decimal} class to perform +modulo calculations and equality tests that are unsuitable for binary +floating point: + +\begin{verbatim} +>>> Decimal('1.00') % Decimal('.10') +Decimal("0.00") +>>> 1.00 % 0.10 +0.09999999999999995 + +>>> sum([Decimal('0.1')]*10) == Decimal('1.0') +True +>>> sum([0.1]*10) == 1.0 +False +\end{verbatim} + +The \module{decimal} module also allows arbitrarily large precisions to be +set for calculation: + +\begin{verbatim} +>>> getcontext().prec = 36 +>>> Decimal(1) / Decimal(7) +Decimal("0.142857142857142857142857142857142857") +\end{verbatim} + + + \chapter{What Now? \label{whatNow}} Reading this tutorial has probably reinforced your interest in using |