decimal — 定点小数

Decimal

# decimal_create.py
import decimal

fmt = '{0:<25} {1:<25}'

print(fmt.format('Input', 'Output'))
print(fmt.format('-' * 25, '-' * 25))

# Integer
print(fmt.format(5, decimal.Decimal(5)))

# String
print(fmt.format('3.14', decimal.Decimal('3.14')))

# Float
f = 0.1
print(fmt.format(repr(f), decimal.Decimal(str(f))))
print('{:<0.23g} {:<25}'.format(
    f,
    str(decimal.Decimal.from_float(f))[:25])
)

# decimal_tuple.py
import decimal

# Tuple
t = (1, (1, 1), -2)
print('Input  :', t)
print('Decimal:', decimal.Decimal(t))

格式化

# decimal_format.py
import decimal

d = decimal.Decimal(1.1)
print('Precision:')
print('{:.1}'.format(d))
print('{:.2}'.format(d))
print('{:.3}'.format(d))
print('{:.18}'.format(d))

print('\nWidth and precision combined:')
print('{:5.1f} {:5.1g}'.format(d, d))
print('{:5.2f} {:5.2g}'.format(d, d))
print('{:5.2f} {:5.2g}'.format(d, d))

print('\nZero padding:')
print('{:05.1}'.format(d))
print('{:05.2}'.format(d))
print('{:05.3}'.format(d))

Arithmetic

# decimal_operators.py
import decimal

a = decimal.Decimal('5.1')
b = decimal.Decimal('3.14')
c = 4
d = 3.14

print('a     =', repr(a))
print('b     =', repr(b))
print('c     =', repr(c))
print('d     =', repr(d))
print()

print('a + b =', a + b)
print('a - b =', a - b)
print('a * b =', a * b)
print('a / b =', a / b)
print()

print('a + c =', a + c)
print('a - c =', a - c)
print('a * c =', a * c)
print('a / c =', a / c)
print()

print('a + d =', end=' ')
try:
    print(a + d)
except TypeError as e:
    print(e)

特殊的数值

# decimal_special.py
import decimal

for value in ['Infinity', 'NaN', '0']:
    print(decimal.Decimal(value), decimal.Decimal('-' + value))
print()

# Math with infinity
print('Infinity + 1:', (decimal.Decimal('Infinity') + 1))
print('-Infinity + 1:', (decimal.Decimal('-Infinity') + 1))

# Print comparing NaN
print(decimal.Decimal('NaN') == decimal.Decimal('Infinity'))
print(decimal.Decimal('NaN') != decimal.Decimal(1))

上下文

当前上下文

# decimal_getcontext.py
import decimal

context = decimal.getcontext()

print('Emax     =', context.Emax)
print('Emin     =', context.Emin)
print('capitals =', context.capitals)
print('prec     =', context.prec)
print('rounding =', context.rounding)
print('flags    =')
for f, v in context.flags.items():
    print('  {}: {}'.format(f, v))
print('traps    =')
for t, v in context.traps.items():
    print('  {}: {}'.format(t, v))

精度

# decimal_precision.py
import decimal

d = decimal.Decimal('0.123456')

for i in range(1, 5):
    decimal.getcontext().prec = i
    print(i, ':', d, d * 1)

Rounding

# decimal_rounding.py
import decimal

context = decimal.getcontext()

ROUNDING_MODES = [
    'ROUND_CEILING',
    'ROUND_DOWN',
    'ROUND_FLOOR',
    'ROUND_HALF_DOWN',
    'ROUND_HALF_EVEN',
    'ROUND_HALF_UP',
    'ROUND_UP',
    'ROUND_05UP',
]

header_fmt = '{:10} ' + ' '.join(['{:^8}'] * 6)

print(header_fmt.format(
    ' ',
    '1/8 (1)', '-1/8 (1)',
    '1/8 (2)', '-1/8 (2)',
    '1/8 (3)', '-1/8 (3)',
))
for rounding_mode in ROUNDING_MODES:
    print('{0:10}'.format(rounding_mode.partition('_')[-1]),
          end=' ')
    for precision in [1, 2, 3]:
        context.prec = precision
        context.rounding = getattr(decimal, rounding_mode)
        value = decimal.Decimal(1) / decimal.Decimal(8)
        print('{0:^8}'.format(value), end=' ')
        value = decimal.Decimal(-1) / decimal.Decimal(8)
        print('{0:^8}'.format(value), end=' ')
    print()

局部上下文

# decimal_context_manager.py
import decimal

with decimal.localcontext() as c:
    c.prec = 2
    print('Local precision:', c.prec)
    print('3.14 / 3 =', (decimal.Decimal('3.14') / 3))

print()
print('Default precision:', decimal.getcontext().prec)
print('3.14 / 3 =', (decimal.Decimal('3.14') / 3))

每个上下文

# decimal_instance_context.py
import decimal

# Set up a context with limited precision
c = decimal.getcontext().copy()
c.prec = 3

# Create our constant
pi = c.create_decimal('3.1415')

# The constant value is rounded off
print('PI    :', pi)

# The result of using the constant uses the global context
print('RESULT:', decimal.Decimal('2.01') * pi)

线程

# decimal_thread_context.py
import decimal
import threading
from queue import PriorityQueue


class Multiplier(threading.Thread):
    def __init__(self, a, b, prec, q):
        self.a = a
        self.b = b
        self.prec = prec
        self.q = q
        threading.Thread.__init__(self)

    def run(self):
        c = decimal.getcontext().copy()
        c.prec = self.prec
        decimal.setcontext(c)
        self.q.put((self.prec, a * b))


a = decimal.Decimal('3.14')
b = decimal.Decimal('1.234')
# A PriorityQueue will return values sorted by precision,
# no matter what order the threads finish.
q = PriorityQueue()
threads = [Multiplier(a, b, i, q) for i in range(1, 6)]
for t in threads:
    t.start()

for t in threads:
    t.join()

for i in range(5):
    prec, value = q.get()
    print('{}  {}'.format(prec, value))