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| 54 | ||
| 55 | <H2><A NAME="SECTION007610000000000000000"></A><A NAME="decimal-tutorial"></A> | |
| 56 | <BR> | |
| 57 | 5.6.1 Quick-start Tutorial | |
| 58 | </H2> | |
| 59 | ||
| 60 | <P> | |
| 61 | The usual start to using decimals is importing the module, viewing the current | |
| 62 | context with <tt class="function">getcontext()</tt> and, if necessary, setting new values | |
| 63 | for precision, rounding, or enabled traps: | |
| 64 | ||
| 65 | <P> | |
| 66 | <div class="verbatim"><pre> | |
| 67 | >>> from decimal import * | |
| 68 | >>> getcontext() | |
| 69 | Context(prec=28, rounding=ROUND_HALF_EVEN, Emin=-999999999, Emax=999999999, | |
| 70 | capitals=1, flags=[], traps=[Overflow, InvalidOperation, | |
| 71 | DivisionByZero]) | |
| 72 | ||
| 73 | >>> getcontext().prec = 7 # Set a new precision | |
| 74 | </pre></div> | |
| 75 | ||
| 76 | <P> | |
| 77 | Decimal instances can be constructed from integers, strings or tuples. To | |
| 78 | create a Decimal from a <tt class="class">float</tt>, first convert it to a string. This | |
| 79 | serves as an explicit reminder of the details of the conversion (including | |
| 80 | representation error). Decimal numbers include special values such as | |
| 81 | <tt class="constant">NaN</tt> which stands for ``Not a number'', positive and negative | |
| 82 | <tt class="constant">Infinity</tt>, and <tt class="constant">-0</tt>. | |
| 83 | ||
| 84 | <P> | |
| 85 | <div class="verbatim"><pre> | |
| 86 | >>> Decimal(10) | |
| 87 | Decimal("10") | |
| 88 | >>> Decimal("3.14") | |
| 89 | Decimal("3.14") | |
| 90 | >>> Decimal((0, (3, 1, 4), -2)) | |
| 91 | Decimal("3.14") | |
| 92 | >>> Decimal(str(2.0 ** 0.5)) | |
| 93 | Decimal("1.41421356237") | |
| 94 | >>> Decimal("NaN") | |
| 95 | Decimal("NaN") | |
| 96 | >>> Decimal("-Infinity") | |
| 97 | Decimal("-Infinity") | |
| 98 | </pre></div> | |
| 99 | ||
| 100 | <P> | |
| 101 | The significance of a new Decimal is determined solely by the number | |
| 102 | of digits input. Context precision and rounding only come into play during | |
| 103 | arithmetic operations. | |
| 104 | ||
| 105 | <P> | |
| 106 | <div class="verbatim"><pre> | |
| 107 | >>> getcontext().prec = 6 | |
| 108 | >>> Decimal('3.0') | |
| 109 | Decimal("3.0") | |
| 110 | >>> Decimal('3.1415926535') | |
| 111 | Decimal("3.1415926535") | |
| 112 | >>> Decimal('3.1415926535') + Decimal('2.7182818285') | |
| 113 | Decimal("5.85987") | |
| 114 | >>> getcontext().rounding = ROUND_UP | |
| 115 | >>> Decimal('3.1415926535') + Decimal('2.7182818285') | |
| 116 | Decimal("5.85988") | |
| 117 | </pre></div> | |
| 118 | ||
| 119 | <P> | |
| 120 | Decimals interact well with much of the rest of python. Here is a small | |
| 121 | decimal floating point flying circus: | |
| 122 | ||
| 123 | <P> | |
| 124 | <div class="verbatim"><pre> | |
| 125 | >>> data = map(Decimal, '1.34 1.87 3.45 2.35 1.00 0.03 9.25'.split()) | |
| 126 | >>> max(data) | |
| 127 | Decimal("9.25") | |
| 128 | >>> min(data) | |
| 129 | Decimal("0.03") | |
| 130 | >>> sorted(data) | |
| 131 | [Decimal("0.03"), Decimal("1.00"), Decimal("1.34"), Decimal("1.87"), | |
| 132 | Decimal("2.35"), Decimal("3.45"), Decimal("9.25")] | |
| 133 | >>> sum(data) | |
| 134 | Decimal("19.29") | |
| 135 | >>> a,b,c = data[:3] | |
| 136 | >>> str(a) | |
| 137 | '1.34' | |
| 138 | >>> float(a) | |
| 139 | 1.3400000000000001 | |
| 140 | >>> round(a, 1) # round() first converts to binary floating point | |
| 141 | 1.3 | |
| 142 | >>> int(a) | |
| 143 | 1 | |
| 144 | >>> a * 5 | |
| 145 | Decimal("6.70") | |
| 146 | >>> a * b | |
| 147 | Decimal("2.5058") | |
| 148 | >>> c % a | |
| 149 | Decimal("0.77") | |
| 150 | </pre></div> | |
| 151 | ||
| 152 | <P> | |
| 153 | The <tt class="method">quantize()</tt> method rounds a number to a fixed exponent. This | |
| 154 | method is useful for monetary applications that often round results to a fixed | |
| 155 | number of places: | |
| 156 | ||
| 157 | <P> | |
| 158 | <div class="verbatim"><pre> | |
| 159 | >>> Decimal('7.325').quantize(Decimal('.01'), rounding=ROUND_DOWN) | |
| 160 | Decimal("7.32") | |
| 161 | >>> Decimal('7.325').quantize(Decimal('1.'), rounding=ROUND_UP) | |
| 162 | Decimal("8") | |
| 163 | </pre></div> | |
| 164 | ||
| 165 | <P> | |
| 166 | As shown above, the <tt class="function">getcontext()</tt> function accesses the current | |
| 167 | context and allows the settings to be changed. This approach meets the | |
| 168 | needs of most applications. | |
| 169 | ||
| 170 | <P> | |
| 171 | For more advanced work, it may be useful to create alternate contexts using | |
| 172 | the Context() constructor. To make an alternate active, use the | |
| 173 | <tt class="function">setcontext()</tt> function. | |
| 174 | ||
| 175 | <P> | |
| 176 | In accordance with the standard, the <tt class="module">Decimal</tt> module provides two | |
| 177 | ready to use standard contexts, <tt class="constant">BasicContext</tt> and | |
| 178 | <tt class="constant">ExtendedContext</tt>. The former is especially useful for debugging | |
| 179 | because many of the traps are enabled: | |
| 180 | ||
| 181 | <P> | |
| 182 | <div class="verbatim"><pre> | |
| 183 | >>> myothercontext = Context(prec=60, rounding=ROUND_HALF_DOWN) | |
| 184 | >>> setcontext(myothercontext) | |
| 185 | >>> Decimal(1) / Decimal(7) | |
| 186 | Decimal("0.142857142857142857142857142857142857142857142857142857142857") | |
| 187 | ||
| 188 | >>> ExtendedContext | |
| 189 | Context(prec=9, rounding=ROUND_HALF_EVEN, Emin=-999999999, Emax=999999999, | |
| 190 | capitals=1, flags=[], traps=[]) | |
| 191 | >>> setcontext(ExtendedContext) | |
| 192 | >>> Decimal(1) / Decimal(7) | |
| 193 | Decimal("0.142857143") | |
| 194 | >>> Decimal(42) / Decimal(0) | |
| 195 | Decimal("Infinity") | |
| 196 | ||
| 197 | >>> setcontext(BasicContext) | |
| 198 | >>> Decimal(42) / Decimal(0) | |
| 199 | Traceback (most recent call last): | |
| 200 | File "<pyshell#143>", line 1, in -toplevel- | |
| 201 | Decimal(42) / Decimal(0) | |
| 202 | DivisionByZero: x / 0 | |
| 203 | </pre></div> | |
| 204 | ||
| 205 | <P> | |
| 206 | Contexts also have signal flags for monitoring exceptional conditions | |
| 207 | encountered during computations. The flags remain set until explicitly | |
| 208 | cleared, so it is best to clear the flags before each set of monitored | |
| 209 | computations by using the <tt class="method">clear_flags()</tt> method. | |
| 210 | ||
| 211 | <P> | |
| 212 | <div class="verbatim"><pre> | |
| 213 | >>> setcontext(ExtendedContext) | |
| 214 | >>> getcontext().clear_flags() | |
| 215 | >>> Decimal(355) / Decimal(113) | |
| 216 | Decimal("3.14159292") | |
| 217 | >>> getcontext() | |
| 218 | Context(prec=9, rounding=ROUND_HALF_EVEN, Emin=-999999999, Emax=999999999, | |
| 219 | capitals=1, flags=[Inexact, Rounded], traps=[]) | |
| 220 | </pre></div> | |
| 221 | ||
| 222 | <P> | |
| 223 | The <var>flags</var> entry shows that the rational approximation to <tt class="constant">Pi</tt> | |
| 224 | was rounded (digits beyond the context precision were thrown away) and that | |
| 225 | the result is inexact (some of the discarded digits were non-zero). | |
| 226 | ||
| 227 | <P> | |
| 228 | Individual traps are set using the dictionary in the <tt class="member">traps</tt> | |
| 229 | field of a context: | |
| 230 | ||
| 231 | <P> | |
| 232 | <div class="verbatim"><pre> | |
| 233 | >>> Decimal(1) / Decimal(0) | |
| 234 | Decimal("Infinity") | |
| 235 | >>> getcontext().traps[DivisionByZero] = 1 | |
| 236 | >>> Decimal(1) / Decimal(0) | |
| 237 | Traceback (most recent call last): | |
| 238 | File "<pyshell#112>", line 1, in -toplevel- | |
| 239 | Decimal(1) / Decimal(0) | |
| 240 | DivisionByZero: x / 0 | |
| 241 | </pre></div> | |
| 242 | ||
| 243 | <P> | |
| 244 | Most programs adjust the current context only once, at the beginning of the | |
| 245 | program. And, in many applications, data is converted to <tt class="class">Decimal</tt> with | |
| 246 | a single cast inside a loop. With context set and decimals created, the bulk | |
| 247 | of the program manipulates the data no differently than with other Python | |
| 248 | numeric types. | |
| 249 | ||
| 250 | <P> | |
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| 286 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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