Make ghostscript default backend and add support for string keywords

camelot/backends/ghostscript_backend.py

@@ -29,8 +29,8 @@ class GhostscriptBackend(object):
     def convert(self, pdf_path, png_path, resolution=300):
         if not self.installed():
             raise OSError(
-                "Ghostscript is not installed. Please install it using the instructions"
-                "here: https://camelot-py.readthedocs.io/en/master/user/install-deps.html"
+                "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

camelot/backends/image_conversion.py

@@ -3,21 +3,21 @@
 from .poppler_backend import PopplerBackend
 from .ghostscript_backend import GhostscriptBackend
 
-backends = {"poppler": PopplerBackend, "ghostscript": GhostscriptBackend}
+BACKENDS = {"poppler": PopplerBackend, "ghostscript": GhostscriptBackend}
 
 
 class ImageConversionBackend(object):
     def __init__(self, backend="poppler", use_fallback=True):
-        if backend not in backends.keys():
+        if backend not in BACKENDS.keys():
             raise ValueError(f"Image conversion backend '{backend}' not supported")
 
         self.backend = backend
         self.use_fallback = use_fallback
-        self.fallbacks = list(filter(lambda x: x != backend, backends.keys()))
+        self.fallbacks = list(filter(lambda x: x != backend, BACKENDS.keys()))
 
     def convert(self, pdf_path, png_path):
         try:
-            converter = backends[self.backend]()
+            converter = BACKENDS[self.backend]()
             converter.convert(pdf_path, png_path)
         except Exception as e:
             import sys
@@ -25,7 +25,7 @@ class ImageConversionBackend(object):
             if self.use_fallback:
                 for fallback in self.fallbacks:
                     try:
-                        converter = backends[fallback]()
+                        converter = BACKENDS[fallback]()
                         converter.convert(pdf_path, png_path)
                     except Exception as e:
                         raise type(e)(

camelot/backends/poppler_backend.py

@@ -9,7 +9,7 @@ class PopplerBackend(object):
         pdftopng_executable = shutil.which("pdftopng")
         if pdftopng_executable is None:
             raise OSError(
-                "pdftopng is not installed. Please install it using the `pip install pdftopng` command."
+                "pdftopng is not installed. You can install it using the 'pip install pdftopng' command."
             )
 
         pdftopng_command = [pdftopng_executable, pdf_path, png_path]

camelot/parsers/lattice.py

@@ -28,7 +28,7 @@ from ..image_processing import (
     find_contours,
     find_joints,
 )
-from ..backends import ImageConversionBackend
+from ..backends.image_conversion import BACKENDS
 
 
 logger = logging.getLogger("camelot")
@@ -111,7 +111,7 @@ class Lattice(BaseParser):
         threshold_constant=-2,
         iterations=0,
         resolution=300,
-        backend=ImageConversionBackend(),
+        backend="ghostscript",
         **kwargs,
     ):
         self.table_regions = table_regions
@@ -129,7 +129,30 @@ class Lattice(BaseParser):
         self.threshold_constant = threshold_constant
         self.iterations = iterations
         self.resolution = resolution
-        self.backend = backend
+        self.backend = Lattice._get_backend(backend)
+
+    @staticmethod
+    def _get_backend(backend):
+        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 in BACKENDS.keys():
+                return BACKENDS[backend]()
+            else:
+                raise NotImplementedError(
+                    f"Unknown backend '{backend}' specified. Please use either 'poppler' or 'ghostscript'."
+                )
+        else:
+            if not implements_convert():
+                raise NotImplementedError(
+                    f"'{backend}' must implement a 'convert' method"
+                )
+
+            return backend
 
     @staticmethod
     def _reduce_index(t, idx, shift_text):