Add abstract base classes for sources and outputs.

src/pricehist/outputs/baseoutput.py

@@ -0,0 +1,7 @@
+from abc import ABC, abstractmethod
+
+
+class BaseOutput(ABC):
+    @abstractmethod
+    def format(self) -> str:
+        pass

src/pricehist/outputs/beancount.py

@@ -1,7 +1,9 @@
 from pricehist.format import Format
 
+from .baseoutput import BaseOutput
 
-class Beancount:
+
+class Beancount(BaseOutput):
     def format(self, series, source=None, fmt=Format()):
         lines = []
         for price in series.prices:

src/pricehist/outputs/csv.py

@@ -1,7 +1,9 @@
 from pricehist.format import Format
 
+from .baseoutput import BaseOutput
 
-class CSV:
+
+class CSV(BaseOutput):
     def format(self, series, source=None, fmt=Format()):
         lines = ["date,base,quote,amount,source,type"]
         for price in series.prices:

src/pricehist/outputs/gnucashsql.py

@@ -5,8 +5,10 @@ from importlib.resources import read_text
 from pricehist import __version__
 from pricehist.format import Format
 
+from .baseoutput import BaseOutput
 
-class GnuCashSQL:
+
+class GnuCashSQL(BaseOutput):
     def format(self, series, source=None, fmt=Format()):
         src = f"pricehist:{source.id()}"
 

src/pricehist/outputs/ledger.py

@@ -1,7 +1,9 @@
 from pricehist.format import Format
 
+from .baseoutput import BaseOutput
 
-class Ledger:
+
+class Ledger(BaseOutput):
     def format(self, series, source=None, fmt=Format()):
         lines = []
         for price in series.prices:

src/pricehist/sources/basesource.py

@@ -0,0 +1,34 @@
+from abc import ABC, abstractmethod
+
+from pricehist.series import Series
+
+
+class BaseSource(ABC):
+    @abstractmethod
+    def id(self) -> str:
+        pass
+
+    @abstractmethod
+    def name(self) -> str:
+        pass
+
+    def description(self) -> str:
+        pass
+
+    def source_url(self) -> str:
+        pass
+
+    def start(self) -> str:
+        pass
+
+    def types(self) -> list[str]:
+        pass
+
+    def notes(self) -> str:
+        pass
+
+    def symbols(self) -> list[str]:
+        pass
+
+    def fetch(self, series: Series) -> Series:
+        pass

src/pricehist/sources/coindesk.py

@@ -6,8 +6,10 @@ import requests
 
 from pricehist.price import Price
 
+from .basesource import BaseSource
 
-class CoinDesk:
+
+class CoinDesk(BaseSource):
     def id(self):
         return "coindesk"