Add typing for camelot/backends

camelot/backends/base.py

@@ -0,0 +1,7 @@
+class ConversionBackend:
+
+    def installed(self) -> bool:
+        raise NotImplementedError
+
+    def convert(self, pdf_path: str, png_path: str, resolution: int = 300) -> None:
+        raise NotImplementedError

camelot/backends/ghostscript_backend.py

@@ -2,36 +2,38 @@
 import sys
 from ctypes.util import find_library
 
+from camelot.backends.base import ConversionBackend
 
-def installed_posix():
+
+def installed_posix() -> bool:
     library = find_library("gs")
     return library is not None
 
 
-def installed_windows():
+def installed_windows() -> bool:
     library = find_library(
-        "".join(("gsdll", str(ctypes.sizeof(ctypes.c_voidp) * 8), ".dll"))
+        "".join(("gsdll", str(ctypes.sizeof(ctypes.c_voidp) * 8), ".dll"))  # type: ignore[attr-defined]
     )
     return library is not None
 
 
-class GhostscriptBackend:
-    def installed(self):
+class GhostscriptBackend(ConversionBackend):
+    def installed(self) -> bool:
         if sys.platform in ["linux", "darwin"]:
             return installed_posix()
         elif sys.platform == "win32":
             return installed_windows()
         else:
             return installed_posix()
 
-    def convert(self, pdf_path, png_path, resolution=300):
+    def convert(self, pdf_path: str, png_path: str, resolution: int = 300) -> None:
         if not self.installed():
             raise OSError(
                 "Ghostscript is not installed. You can install it using the instructions"
                 " here: https://camelot-py.readthedocs.io/en/master/user/install-deps.html"
             )
 
-        import ghostscript
+        import ghostscript  # type: ignore[import-untyped]
 
         gs_command = [
             "gs",

camelot/backends/image_conversion.py

@@ -1,39 +1,49 @@
+from typing import Dict
+from typing import List
+from typing import Type
+
+from .base import ConversionBackend
 from .ghostscript_backend import GhostscriptBackend
 from .poppler_backend import PopplerBackend
 
 
-BACKENDS = {"poppler": PopplerBackend, "ghostscript": GhostscriptBackend}
+BACKENDS: Dict[str, Type[ConversionBackend]] = {
+    "poppler": PopplerBackend,
+    "ghostscript": GhostscriptBackend,
+}
+
+
+class ImageConversionError(ValueError):
+    pass
 
 
 class ImageConversionBackend:
-    def __init__(self, backend="poppler", use_fallback=True):
+    def __init__(self, backend: str = "poppler", use_fallback: bool = True) -> None:
         if backend not in BACKENDS.keys():
-            raise ValueError(f"Image conversion backend '{backend}' not supported")
+            msg = f"Image conversion backend {backend!r} not supported"
+            raise ValueError(msg)
 
-        self.backend = backend
-        self.use_fallback = use_fallback
-        self.fallbacks = list(filter(lambda x: x != backend, BACKENDS.keys()))
+        self.backend: str = backend
+        self.use_fallback: bool = use_fallback
+        self.fallbacks: List[str] = list(
+            filter(lambda x: x != backend, BACKENDS.keys())
+        )
 
-    def convert(self, pdf_path, png_path):
+    def convert(self, pdf_path: str, png_path: str) -> None:
         try:
             converter = BACKENDS[self.backend]()
             converter.convert(pdf_path, png_path)
-        except Exception as e:
-            import sys
-
+        except Exception as f:
             if self.use_fallback:
                 for fallback in self.fallbacks:
                     try:
                         converter = BACKENDS[fallback]()
                         converter.convert(pdf_path, png_path)
                     except Exception as e:
-                        raise type(e)(
-                            str(e) + f" with image conversion backend '{fallback}'"
-                        ).with_traceback(sys.exc_info()[2])
-                        continue
+                        msg = f"Image conversion failed with image conversion backend {fallback!r}"
+                        raise ImageConversionError(msg) from e
                     else:
                         break
             else:
-                raise type(e)(
-                    str(e) + f" with image conversion backend '{self.backend}'"
-                ).with_traceback(sys.exc_info()[2])
+                msg = f"Image conversion failed with image conversion backend {self.backend!r}"
+                raise ImageConversionError(msg) from f

camelot/backends/poppler_backend.py

@@ -1,12 +1,16 @@
 import os
-import sys
 import shutil
 import subprocess
+import sys
+
+from camelot.backends.base import ConversionBackend
+
+
+path = os.path.dirname(sys.executable) + os.pathsep + os.environ["PATH"]
 
-path = os.path.dirname(sys.executable) + os.pathsep + os.environ['PATH']
 
-class PopplerBackend:
-    def convert(self, pdf_path, png_path):
+class PopplerBackend(ConversionBackend):
+    def convert(self, pdf_path: str, png_path: str, resolution: int = 300) -> None:
         pdftopng_executable = shutil.which("pdftopng", path=path)
         if pdftopng_executable is None:
             raise OSError(
@@ -20,4 +24,4 @@ def convert(self, pdf_path, png_path):
                 " ".join(pdftopng_command), stderr=subprocess.STDOUT, shell=True
             )
         except subprocess.CalledProcessError as e:
-            raise ValueError(e.output)
+            raise ValueError(e.output) from e