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103 lines
3.1 KiB
Python
103 lines
3.1 KiB
Python
"""
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``python-future``: pure Python implementation of Python 3 round().
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"""
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from future.utils import PYPY, PY26, bind_method
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# Use the decimal module for simplicity of implementation (and
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# hopefully correctness).
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from decimal import Decimal, ROUND_HALF_EVEN
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def newround(number, ndigits=None):
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"""
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See Python 3 documentation: uses Banker's Rounding.
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Delegates to the __round__ method if for some reason this exists.
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If not, rounds a number to a given precision in decimal digits (default
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0 digits). This returns an int when called with one argument,
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otherwise the same type as the number. ndigits may be negative.
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See the test_round method in future/tests/test_builtins.py for
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examples.
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"""
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return_int = False
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if ndigits is None:
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return_int = True
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ndigits = 0
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if hasattr(number, '__round__'):
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return number.__round__(ndigits)
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if ndigits < 0:
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raise NotImplementedError('negative ndigits not supported yet')
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exponent = Decimal('10') ** (-ndigits)
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if PYPY:
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# Work around issue #24: round() breaks on PyPy with NumPy's types
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if 'numpy' in repr(type(number)):
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number = float(number)
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if isinstance(number, Decimal):
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d = number
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else:
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if not PY26:
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d = Decimal.from_float(number).quantize(exponent,
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rounding=ROUND_HALF_EVEN)
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else:
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d = from_float_26(number).quantize(exponent, rounding=ROUND_HALF_EVEN)
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if return_int:
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return int(d)
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else:
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return float(d)
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### From Python 2.7's decimal.py. Only needed to support Py2.6:
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def from_float_26(f):
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"""Converts a float to a decimal number, exactly.
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Note that Decimal.from_float(0.1) is not the same as Decimal('0.1').
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Since 0.1 is not exactly representable in binary floating point, the
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value is stored as the nearest representable value which is
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0x1.999999999999ap-4. The exact equivalent of the value in decimal
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is 0.1000000000000000055511151231257827021181583404541015625.
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>>> Decimal.from_float(0.1)
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Decimal('0.1000000000000000055511151231257827021181583404541015625')
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>>> Decimal.from_float(float('nan'))
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Decimal('NaN')
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>>> Decimal.from_float(float('inf'))
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Decimal('Infinity')
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>>> Decimal.from_float(-float('inf'))
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Decimal('-Infinity')
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>>> Decimal.from_float(-0.0)
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Decimal('-0')
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"""
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import math as _math
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from decimal import _dec_from_triple # only available on Py2.6 and Py2.7 (not 3.3)
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if isinstance(f, (int, long)): # handle integer inputs
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return Decimal(f)
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if _math.isinf(f) or _math.isnan(f): # raises TypeError if not a float
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return Decimal(repr(f))
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if _math.copysign(1.0, f) == 1.0:
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sign = 0
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else:
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sign = 1
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n, d = abs(f).as_integer_ratio()
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# int.bit_length() method doesn't exist on Py2.6:
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def bit_length(d):
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if d != 0:
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return len(bin(abs(d))) - 2
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else:
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return 0
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k = bit_length(d) - 1
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result = _dec_from_triple(sign, str(n*5**k), -k)
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return result
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__all__ = ['newround']
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