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<H2><A NAME=
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5.6.1 Quick-start Tutorial
The usual start to using decimals is importing the module, viewing the current
context with
<tt class=
"function">getcontext()
</tt> and, if necessary, setting new values
for precision, rounding, or enabled traps:
<div class=
"verbatim"><pre>
>>> from decimal import *
>>> getcontext()
Context(prec=
28, rounding=ROUND_HALF_EVEN, Emin=-
999999999, Emax=
999999999,
capitals=
1, flags=[], traps=[Overflow, InvalidOperation,
>>> getcontext().prec =
7 # Set a new precision
Decimal instances can be constructed from integers, strings or tuples. To
create a Decimal from a
<tt class=
"class">float
</tt>, first convert it to a string. This
serves as an explicit reminder of the details of the conversion (including
representation error). Decimal numbers include special values such as
<tt class=
"constant">NaN
</tt> which stands for ``Not a number'', positive and negative
<tt class=
"constant">Infinity
</tt>, and
<tt class=
"constant">-
0</tt>.
<div class=
"verbatim"><pre>
>>> Decimal(
"3.14")
>>> Decimal((
0, (
3,
1,
4), -
2))
>>> Decimal(str(
2.0 **
0.5))
>>> Decimal(
"NaN")
>>> Decimal(
"-Infinity")
The significance of a new Decimal is determined solely by the number
of digits input. Context precision and rounding only come into play during
<div class=
"verbatim"><pre>
>>> getcontext().prec =
6
>>> Decimal('
3.0')
>>> Decimal('
3.1415926535')
>>> Decimal('
3.1415926535') + Decimal('
2.7182818285')
>>> getcontext().rounding = ROUND_UP
>>> Decimal('
3.1415926535') + Decimal('
2.7182818285')
Decimals interact well with much of the rest of python. Here is a small
decimal floating point flying circus:
<div class=
"verbatim"><pre>
>>> data = map(Decimal, '
1.34 1.87 3.45 2.35 1.00 0.03 9.25'.split())
>>> sorted(data)
[Decimal(
"0.03"), Decimal(
"1.00"), Decimal(
"1.34"), Decimal(
"1.87"),
Decimal(
"2.35"), Decimal(
"3.45"), Decimal(
"9.25")]
>>> a,b,c = data[:
3]
>>> round(a,
1) # round() first converts to binary floating point
The
<tt class=
"method">quantize()
</tt> method rounds a number to a fixed exponent. This
method is useful for monetary applications that often round results to a fixed
<div class=
"verbatim"><pre>
>>> Decimal('
7.325').quantize(Decimal('
.01'), rounding=ROUND_DOWN)
>>> Decimal('
7.325').quantize(Decimal('
1.'), rounding=ROUND_UP)
As shown above, the
<tt class=
"function">getcontext()
</tt> function accesses the current
context and allows the settings to be changed. This approach meets the
needs of most applications.
For more advanced work, it may be useful to create alternate contexts using
the Context() constructor. To make an alternate active, use the
<tt class=
"function">setcontext()
</tt> function.
In accordance with the standard, the
<tt class=
"module">Decimal
</tt> module provides two
ready to use standard contexts,
<tt class=
"constant">BasicContext
</tt> and
<tt class=
"constant">ExtendedContext
</tt>. The former is especially useful for debugging
because many of the traps are enabled:
<div class=
"verbatim"><pre>
>>> myothercontext = Context(prec=
60, rounding=ROUND_HALF_DOWN)
>>> setcontext(myothercontext)
>>> Decimal(
1) / Decimal(
7)
Decimal(
"0.142857142857142857142857142857142857142857142857142857142857")
>>> ExtendedContext
Context(prec=
9, rounding=ROUND_HALF_EVEN, Emin=-
999999999, Emax=
999999999,
capitals=
1, flags=[], traps=[])
>>> setcontext(ExtendedContext)
>>> Decimal(
1) / Decimal(
7)
>>> Decimal(
42) / Decimal(
0)
>>> setcontext(BasicContext)
>>> Decimal(
42) / Decimal(
0)
Traceback (most recent call last):
File
"<pyshell#143>", line
1, in -toplevel-
Contexts also have signal flags for monitoring exceptional conditions
encountered during computations. The flags remain set until explicitly
cleared, so it is best to clear the flags before each set of monitored
computations by using the
<tt class=
"method">clear_flags()
</tt> method.
<div class=
"verbatim"><pre>
>>> setcontext(ExtendedContext)
>>> getcontext().clear_flags()
>>> Decimal(
355) / Decimal(
113)
>>> getcontext()
Context(prec=
9, rounding=ROUND_HALF_EVEN, Emin=-
999999999, Emax=
999999999,
capitals=
1, flags=[Inexact, Rounded], traps=[])
The
<var>flags
</var> entry shows that the rational approximation to
<tt class=
"constant">Pi
</tt>was rounded (digits beyond the context precision were thrown away) and that
the result is inexact (some of the discarded digits were non-zero).
Individual traps are set using the dictionary in the
<tt class=
"member">traps
</tt><div class=
"verbatim"><pre>
>>> Decimal(
1) / Decimal(
0)
>>> getcontext().traps[DivisionByZero] =
1
>>> Decimal(
1) / Decimal(
0)
Traceback (most recent call last):
File
"<pyshell#112>", line
1, in -toplevel-
Most programs adjust the current context only once, at the beginning of the
program. And, in many applications, data is converted to
<tt class=
"class">Decimal
</tt> with
a single cast inside a loop. With context set and decimals created, the bulk
of the program manipulates the data no differently than with other Python
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<span class=
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2.4.2, documentation updated on
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2005.
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