Simplify and fix Table.set_span

.github/workflows/constraints.txt

@@ -3,4 +3,4 @@ pip==24.2
 nox==2024.10.9
 nox-poetry==1.0.3
 poetry>=1.2.0
-virtualenv==20.26.6
+virtualenv==20.27.0

camelot/backends/image_conversion.py

@@ -1,5 +1,6 @@
 """Classes and functions for the ImageConversionBackend backends."""
 
+from typing import Any
 from typing import Dict
 from typing import List
 from typing import Type
@@ -22,7 +23,7 @@ class ImageConversionError(ValueError):  # noqa D101
 class ImageConversionBackend:
     """Classes the ImageConversionBackend backend."""
 
-    def __init__(self, backend: str = "poppler", use_fallback: bool = True) -> None:
+    def __init__(self, backend: Any = "poppler", use_fallback: bool = True) -> None:
         """Initialize the conversion backend .
 
         Parameters
@@ -37,15 +38,70 @@ def __init__(self, backend: str = "poppler", use_fallback: bool = True) -> None:
         ValueError
             Raise an error if the backend is not supported.
         """
-        if backend not in BACKENDS.keys():
-            raise ValueError(f"Image conversion backend {backend!r} not supported")
-
-        self.backend: str = backend
+        self.backend: ConversionBackend = self.get_backend(backend)
         self.use_fallback: bool = use_fallback
         self.fallbacks: List[str] = list(
-            filter(lambda x: x != backend, BACKENDS.keys())
+            filter(lambda x: isinstance(backend, str) and x != backend, BACKENDS.keys())
         )
 
+    def get_backend(self, backend):
+        """Retrieve the specified backend for processing.
+
+        This method checks if the provided backend is a string representing
+        a known backend or an object implementing a 'convert' method. It
+        returns an instance of the backend if valid.
+
+        Parameters
+        ----------
+        backend : str or object
+            The backend to retrieve. This can be:
+                - A string ('poppler' or 'ghostscript') corresponding to a pre-defined backend.
+                - An object that must implement a 'convert' method.
+
+        Returns
+        -------
+        object
+            An instance of the specified backend.
+
+        Raises
+        ------
+        NotImplementedError
+            If the backend is a string that is not recognized or if it is an
+            object that does not implement the 'convert' method.
+
+        Examples
+        --------
+        >> backend_instance = get_backend('poppler')
+        >> backend_instance = get_backend(my_custom_backend)
+
+        Notes
+        -----
+        The valid backends are defined in the BACKENDS dictionary. The
+        method verifies the backend type and raises exceptions for
+        unsupported backends.
+        """
+
+        def implements_convert():
+            methods = [
+                method for method in dir(backend) if method.startswith("__") is False
+            ]
+            return "convert" in methods
+
+        if isinstance(backend, str):
+            if backend not in BACKENDS.keys():
+                raise NotImplementedError(
+                    f"Unknown backend {backend!r} specified. Please use either 'poppler' or 'ghostscript'."
+                )
+
+            return BACKENDS[backend]()
+        else:
+            if not implements_convert():
+                raise NotImplementedError(
+                    f"{backend!r} must implement a 'convert' method"
+                )
+
+            return backend
+
     def convert(self, pdf_path: str, png_path: str) -> None:
         """Convert PDF to png_path.
 
@@ -64,8 +120,7 @@ def convert(self, pdf_path: str, png_path: str) -> None:
             [description]
         """
         try:
-            converter = BACKENDS[self.backend]()
-            converter.convert(pdf_path, png_path)
+            self.backend.convert(pdf_path, png_path)
         except Exception as f:
             if self.use_fallback:
                 for fallback in self.fallbacks: