Implement binary format codec for numeric type

[elprans]

Fixes: #157

[1st1]

Python also supports signalling NaN (sNaN) -- add a check that we handle it with a proper exception. Or maybe we should convert Python's sNaN to pg NaN.

[1st1]

Other than a sNaN thing, lgtm.

[sergeyspatar]

I tested this version on a bunch of different values and tests passed:

import asyncpg
import asyncio

#import uvloop
#asyncio.set_event_loop(uvloop.new_event_loop())

loop = asyncio.get_event_loop()

from decimal import Decimal

async def run(value):
    conn = await asyncpg.connect(user='postgres')
    await conn.execute('create temporary table temp_table (c1 numeric(6,3))')
    await conn.copy_records_to_table('temp_table', records=[(value,)])
    return await conn.fetchrow('select c1 from temp_table')

for value in [123, -999, 0.123, -999.999, Decimal('0.123'), Decimal('-0.001'), Decimal('-0.0009'), None]:
    print('Testing:', repr(value))
    row = loop.run_until_complete(run(value))
    print('Result:', repr(row[0]))
    print()

Output:

Testing: 123
Result: Decimal('123')

Testing: -999
Result: Decimal('-999')

Testing: 0.123
Result: Decimal('0.1230')

Testing: -999.999
Result: Decimal('-999.9990')

Testing: Decimal('0.123')
Result: Decimal('0.1230')

Testing: Decimal('-0.001')
Result: Decimal('-0.0010')

Testing: Decimal('-0.0009')
Result: Decimal('-0.0010')

Testing: None
Result: None

The only thing I noticed is that when selecting column back from table returned Decimal value has higher precision than in column type (4 vs. 3 digits after decimal point in my tests). For example, Decimal('0.123') saved as numeric(6,3) and then queried became Decimal('0.1230').

[elprans]

The only thing I noticed is that when selecting column back from table returned Decimal value has higher precision than in column type (4 vs. 3 digits after decimal point in my tests). For example, Decimal('0.123') saved as numeric(6,3) and then queried became Decimal('0.1230').

Thanks for catching this. I pushed an updated version that fixes this.

[sergeyspatar]

I pushed an updated version that fixes this.

Thank you Elvis! I retested and now fetched Decimal precision is correct.