src/budget_sync/money.py

   1 LOG10_CENTS_PER_EURO = 2
   2 CENTS_PER_EURO = 10 ** LOG10_CENTS_PER_EURO
   3
   4 __all__ = ["Money", "LOG10_CENTS_PER_EURO", "CENTS_PER_EURO"]
   5
   6
   7 def _is_ascii_digits(s: str):
   8     try:
   9         s.encode("ascii")
  10     except UnicodeEncodeError:
  11         return False
  12     return s.isdigit()
  13
  14
  15 class Money:
  16     """class for handling money, stored as an integer number of cents.
  17
  18     Note: float is not appropriate for dealing with monetary values due to
  19     loss of precision and round-off error. Decimal has similar issues, but to
  20     a smaller extent."""
  21
  22     cents: int
  23
  24     __slots__ = ["cents"]
  25
  26     def __init__(self, value=None, *, cents=None):
  27         if value is None:
  28             if cents is None:
  29                 cents = 0
  30             assert isinstance(cents, int)
  31         elif isinstance(value, Money):
  32             cents = value.cents
  33         elif isinstance(value, int):
  34             cents = value * CENTS_PER_EURO
  35         elif isinstance(value, float):
  36             raise TypeError("float is not an appropriate type"
  37                             " for dealing with money")
  38         else:
  39             cents = self.from_str(value).cents
  40         self.cents = cents
  41
  42     @staticmethod
  43     def from_str(text: str):
  44         if not isinstance(text, str):
  45             raise TypeError("Can't use Money.from_str to "
  46                             "convert from non-str value")
  47         parts = text.strip().split(".", maxsplit=2)
  48         first_part = parts[0]
  49         negative = first_part.startswith("-")
  50         if first_part.startswith(("-", "+")):
  51             first_part = first_part[1:]
  52         if first_part == "":
  53             euros = 0
  54         elif _is_ascii_digits(first_part):
  55             euros = int(first_part)
  56         else:
  57             raise ValueError("invalid Money string: characters after sign and"
  58                              " before first `.` must be ascii digits")
  59         if len(parts) > 2:
  60             raise ValueError("invalid Money string: too many `.` characters")
  61         elif len(parts) == 2:
  62             if parts[1] == "":
  63                 if first_part == "":
  64                     raise ValueError("invalid Money string: missing digits")
  65                 cents = 0
  66             elif _is_ascii_digits(parts[1]):
  67                 shift_amount = LOG10_CENTS_PER_EURO - len(parts[1])
  68                 if shift_amount < 0:
  69                     raise ValueError("invalid Money string: too many digits"
  70                                      " after `.`")
  71                 cents = int(parts[1]) * (10 ** shift_amount)
  72             else:
  73                 raise ValueError("invalid Money string: characters"
  74                                  " after `.` must be ascii digits")
  75         elif first_part == "":
  76             raise ValueError("invalid Money string: missing digits")
  77         else:
  78             cents = 0
  79         cents += CENTS_PER_EURO * euros
  80         if negative:
  81             cents = -cents
  82         return Money(cents=cents)
  83
  84     def __str__(self):
  85         retval = "-" if self.cents < 0 else ""
  86         retval += str(abs(self.cents) // CENTS_PER_EURO)
  87         cents = abs(self.cents) % CENTS_PER_EURO
  88         if cents != 0:
  89             retval += "."
  90             retval += str(cents).zfill(LOG10_CENTS_PER_EURO)
  91         return retval
  92
  93     def __lt__(self, other):
  94         return self.cents < Money(other).cents
  95
  96     def __le__(self, other):
  97         return self.cents <= Money(other).cents
  98
  99     def __eq__(self, other):
 100         return self.cents == Money(other).cents
 101
 102     def __ne__(self, other):
 103         return self.cents != Money(other).cents
 104
 105     def __gt__(self, other):
 106         return self.cents > Money(other).cents
 107
 108     def __ge__(self, other):
 109         return self.cents >= Money(other).cents
 110
 111     def __repr__(self):
 112         return f"Money({repr(str(self))})"
 113
 114     def __bool__(self):
 115         return bool(self.cents)
 116
 117     def __add__(self, other):
 118         cents = self.cents + Money(other).cents
 119         return Money(cents=cents)
 120
 121     def __radd__(self, other):
 122         cents = Money(other).cents + self.cents
 123         return Money(cents=cents)
 124
 125     def __sub__(self, other):
 126         cents = self.cents - Money(other).cents
 127         return Money(cents=cents)
 128
 129     def __rsub__(self, other):
 130         cents = Money(other).cents - self.cents
 131         return Money(cents=cents)
 132
 133     def __mul__(self, other):
 134         if not isinstance(other, int):
 135             raise TypeError("can't multiply by non-int")
 136         cents = self.cents * other
 137         return Money(cents=cents)
 138
 139     def __rmul__(self, other):
 140         if not isinstance(other, int):
 141             raise TypeError("can't multiply by non-int")
 142         cents = other * self.cents
 143         return Money(cents=cents)