fixfield 0.1.0

Fixed-decimal arithmetic with per-field precision enforcement

fixfield

Fixed-decimal arithmetic for Python with per-field precision enforcement.

Inspired by COBOL's PIC clause — declare precision once on the field, and it is enforced automatically on every assignment and arithmetic result.

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Installation

uv add fixfield
# or
pip install fixfield

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Quick Start

from fixfield import Record, Field, RoundingStrategy

class Invoice(Record):
    price    = Field(places=2)
    tax_rate = Field(places=4)
    tax      = Field(places=2)
    total    = Field(places=2)

inv = Invoice(price="19.99", tax_rate="0.0825")
inv.tax   = inv.price * inv.tax_rate   # 1.649175 → rounded to 1.65
inv.total = inv.price + inv.tax        # 21.64

print(inv.total)   # "21.64"
print(repr(inv))   # Invoice(price=19.99, tax_rate=0.0825, tax=1.65, total=21.64)

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Why Not Just Use decimal.Decimal?

	decimal.Decimal	fixfield
Precision location	Global context or per .quantize() call	Declared on the field, enforced automatically
Rounding enforcement	Manual on every result	Automatic on every assignment
Per-field rounding strategy	Manual	Declarative
Domain modelling	Plain values	Named record schema

With decimal you must call .quantize() on every result or silently lose precision. With fixfield the field declaration is the single source of truth.

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Core Concepts

FixedDecimal

A scalar decimal value locked to a declared precision.

from fixfield import FixedDecimal, RoundingStrategy

price = FixedDecimal("19.999", places=2)
str(price)   # "20.00" — rounded on construction

# Arithmetic preserves left operand's precision
result = price + FixedDecimal("1.005", places=4)
result.places     # 2  (left operand wins)
str(result)       # "21.00"

# Comparisons work naturally
price > FixedDecimal("10.00")   # True
price == "20.00"                # True

# Unary operators
str(-price)        # "-20.00"
str(abs(-price))   # "20.00"

Float inputs are automatically converted via str to avoid binary imprecision:

FixedDecimal(0.1 + 0.2, places=2)   # "0.30" not "0.30000000000000004"

Field

A descriptor that enforces precision on a class attribute. Used inside a Record.

from fixfield import Field, RoundingStrategy

price    = Field(places=2)                                    # default ROUND_HALF_UP
tax_rate = Field(places=4, rounding=RoundingStrategy.ROUND_FLOOR)
total    = Field(places=2, default="0.00")
capped   = Field(places=2, digits=5)                          # max 99999.99

Parameter	Default	Description
places	2	Decimal places to keep
rounding	ROUND_HALF_UP	Rounding strategy on assignment
default	None	Default value (zero if not set)
digits	None	Max integer digits — raises FieldOverflowError if exceeded

Record

A structured collection of Field descriptors. Generates __init__, __repr__, and __eq__ automatically.

Field ordering relies on Python's guaranteed dict insertion order (Python 3.7+). Fields are serialised to and from fixed-width strings in the order they are declared in the class body.

from fixfield import Record, Field

class Payment(Record):
    amount     = Field(places=2, digits=7)   # up to 9999999.99
    fee        = Field(places=2, digits=4)
    net        = Field(places=2, digits=7)

p = Payment(amount="1000.00", fee="2.50")
p.net = p.amount - p.fee

print(p)          # Payment(amount=1000.00, fee=2.50, net=997.50)
p.to_dict()       # {"amount": FixedDecimal(...), "fee": ..., "net": ...}

RecordField

Embed a nested Record as a field inside another Record. The nested record's fields participate in to_string/from_string as a contiguous block.

from fixfield import Record, Field, RecordField

class Address(Record):
    zip_code = Field(places=0, digits=5)
    state    = Field(places=0, digits=2)

class Customer(Record):
    customer_id = Field(places=0, digits=6)
    address     = RecordField(Address)

c = Customer(customer_id="42", address=Address(zip_code="90210", state="6"))
line = c.to_string()                        # "    42 90210 6"
parsed = Customer.from_string(line)
str(parsed.address.zip_code)                # "90210"

RecordField is generic: RecordField[Address] so your IDE knows that c.address is an Address, not just a Record.

RoundingStrategy

from fixfield import RoundingStrategy

RoundingStrategy.ROUND_HALF_UP    # 2.5 → 3  (COBOL ROUNDED default)
RoundingStrategy.ROUND_HALF_DOWN  # 2.5 → 2
RoundingStrategy.ROUND_HALF_EVEN  # 2.5 → 2, 3.5 → 4  (banker's rounding)
RoundingStrategy.ROUND_HALF_ODD   # 2.5 → 3, 3.5 → 3
RoundingStrategy.ROUND_UP         # always away from zero
RoundingStrategy.ROUND_DOWN       # always toward zero (truncate)
RoundingStrategy.ROUND_CEILING    # toward +∞
RoundingStrategy.ROUND_FLOOR      # toward -∞

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Fixed-Width Serialization

When every Field has digits set, records can be serialized to and from fixed-width strings — useful for mainframe flat files and legacy EDI formats.

class CustomerRecord(Record):
    customer_id = Field(places=0, digits=6)    # 7 chars:  " 123456"
    balance     = Field(places=2, digits=8)    # 11 chars: "  99999.99"

rec = CustomerRecord(customer_id="123456", balance="99999.99")
line = rec.to_string()             # " 123456  99999.99"

parsed = CustomerRecord.from_string(line)
parsed.balance == rec.balance      # True

Field width formula: 1 (sign) + digits + (1 + places if places > 0 else 0)

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Overflow Protection

from fixfield import Field, Record, FieldOverflowError

class Account(Record):
    balance = Field(places=2, digits=5)   # max 99999.99

a = Account()
a.balance = "99999.99"   # ok
a.balance = "100000.00"  # raises FieldOverflowError

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Dataclass Integration

For users who prefer @dataclass, use FixedDecimal directly and coerce values in __post_init__:

from dataclasses import dataclass, field
from fixfield import FixedDecimal, RoundingStrategy

@dataclass
class LineItem:
    price:    FixedDecimal = field(default_factory=lambda: FixedDecimal(0, places=2))
    quantity: int = 1

    def __post_init__(self):
        if not isinstance(self.price, FixedDecimal):
            self.price = FixedDecimal(self.price, places=2)

    @property
    def total(self) -> FixedDecimal:
        return self.price * self.quantity

For full precision enforcement without __post_init__ boilerplate, use Record instead.

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License

MIT