Added string_id format

fin_defs/__init__.py

@@ -45,6 +45,11 @@ class Asset:
         return COMMON_NAMES.get(self, [])
 
     def to_polygon_id(self) -> str:
+        """Formats this asset to the [`polygon.io`](https://polygon.io)
+        identifier format.
+
+        Inverse of `from_polygon_id`.
+        """
         # TODO: Support currency pairs not involving USD.
         if isinstance(self, Stock):
             return f'{self.ticker}'
@@ -61,6 +66,11 @@ class Asset:
 
     @staticmethod
     def from_polygon_id(polygon_id: str) -> 'Asset':
+        """Parses an asset from the [`polygon.io`](https://polygon.io)
+        identifier format.
+
+        Inverse of `to_polygon_id`.
+        """
         if polygon_id.startswith('I:'):
             return Index(polygon_id.removeprefix('I:'))
         if polygon_id.startswith('X:'):
@@ -72,6 +82,46 @@ class Asset:
             return FiatCurrency(polygon_id.removeprefix('C:').removesuffix('USD'))
         return Stock.from_polygon_id(polygon_id)
 
+    def to_string_id(self) -> str:
+        """Formats the asset id using the fin_defs asset format."""
+        if isinstance(self, Stock):
+            return f'stock:{self}'
+        if isinstance(self, FiatCurrency):
+            return f'fiat:{self}'
+        if isinstance(self, CryptoCurrency):
+            return f'crypto:{self}'
+        if isinstance(self, Index):
+            return f'index:{self}'
+
+        raise NotImplementedError('Unsupported asset type: ' + str(self))
+
+    @staticmethod
+    def from_string_id(asset_id: str) -> 'Asset':
+        """Parses an `Asset` using the fin_defs asset format."""
+        if ':' in asset_id:
+            prefix, ticker = asset_id.split(':')
+        else:
+            prefix, ticker = None, asset_id
+
+        if known_symbol := WELL_KNOWN_SYMBOLS.get(ticker, None):
+            return known_symbol
+
+        if prefix == 'stock':
+            if m := re.match(r'^(\w+)(?:\.(\w+))?$', ticker):
+                exchange = EXCHANGES_BY_IDS[m.group(2) or 'NYSE']  # TODO?
+                return Stock(m.group(1), exchange)
+        if prefix == 'currency':
+            return FiatCurrency(ticker)
+        if prefix == 'fiat':
+            return FiatCurrency(ticker)
+        if prefix == 'index':
+            return Index(ticker)
+        if prefix == 'crypto':
+            return CryptoCurrency(ticker, None)  # TODO
+
+        msg = f'Unsupported asset format: {asset_id}'
+        raise NotImplementedError(msg)
+
 
 @enforce_typing.enforce_types
 @dataclasses.dataclass(frozen=True)