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"
+                "Ghostscript is not installed. You can install it using the instructions"
-                "here: https://camelot-py.readthedocs.io/en/master/user/install-deps.html"
+                " 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):

docs/user/advanced.rst

@@ -644,8 +644,5 @@ When using the :ref:`Lattice <lattice>` flavor, Camelot uses `pdftopng <https://
 
 In case you want to be explicit about the image conversion backend that Camelot should use, you can supply them like this::
 
-    >>> from camelot.backends.poppler_backend import PopplerBackend
+    >>> tables = camelot.read_pdf(filename, backend="poppler")
-    >>> from camelot.backends.ghostscript_backend import GhostscriptBackend
+    >>> tables = camelot.read_pdf(filename, backend="ghostscript")
-    >>>
-    >>> tables = camelot.read_pdf(filename, backend=PopplerBackend())
-    >>> tables = camelot.read_pdf(filename, backend=GhostscriptBackend())

tests/test_errors.py

@@ -32,56 +32,6 @@ def test_unsupported_format():
         tables = camelot.read_pdf(filename)
 
 
-def test_stream_equal_length():
-    message = "Length of table_areas and columns" " should be equal"
-    with pytest.raises(ValueError, match=message):
-        tables = camelot.read_pdf(
-            filename,
-            flavor="stream",
-            table_areas=["10,20,30,40"],
-            columns=["10,20,30,40", "10,20,30,40"],
-        )
-
-
-def test_image_warning():
-    filename = os.path.join(testdir, "image.pdf")
-    with warnings.catch_warnings():
-        warnings.simplefilter("error")
-        with pytest.raises(UserWarning) as e:
-            tables = camelot.read_pdf(filename)
-            assert (
-                str(e.value)
-                == "page-1 is image-based, camelot only works on text-based pages."
-            )
-
-
-def test_lattice_no_tables_on_page():
-    filename = os.path.join(testdir, "empty.pdf")
-    with warnings.catch_warnings():
-        warnings.simplefilter("error")
-        with pytest.raises(UserWarning) as e:
-            tables = camelot.read_pdf(filename, flavor="lattice")
-        assert str(e.value) == "No tables found on page-1"
-
-
-def test_stream_no_tables_on_page():
-    filename = os.path.join(testdir, "empty.pdf")
-    with warnings.catch_warnings():
-        warnings.simplefilter("error")
-        with pytest.raises(UserWarning) as e:
-            tables = camelot.read_pdf(filename, flavor="stream")
-        assert str(e.value) == "No tables found on page-1"
-
-
-def test_stream_no_tables_in_area():
-    filename = os.path.join(testdir, "only_page_number.pdf")
-    with warnings.catch_warnings():
-        warnings.simplefilter("error")
-        with pytest.raises(UserWarning) as e:
-            tables = camelot.read_pdf(filename, flavor="stream")
-        assert str(e.value) == "No tables found in table area 1"
-
-
 def test_no_tables_found_logs_suppressed():
     filename = os.path.join(testdir, "foo.pdf")
     with warnings.catch_warnings():
@@ -118,3 +68,68 @@ def test_bad_password():
     message = "file has not been decrypted"
     with pytest.raises(Exception, match=message):
         tables = camelot.read_pdf(filename, password="wrongpass")
+
+
+def test_stream_equal_length():
+    message = "Length of table_areas and columns" " should be equal"
+    with pytest.raises(ValueError, match=message):
+        tables = camelot.read_pdf(
+            filename,
+            flavor="stream",
+            table_areas=["10,20,30,40"],
+            columns=["10,20,30,40", "10,20,30,40"],
+        )
+
+
+def test_image_warning():
+    filename = os.path.join(testdir, "image.pdf")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        with pytest.raises(UserWarning) as e:
+            tables = camelot.read_pdf(filename)
+            assert (
+                str(e.value)
+                == "page-1 is image-based, camelot only works on text-based pages."
+            )
+
+
+def test_stream_no_tables_on_page():
+    filename = os.path.join(testdir, "empty.pdf")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        with pytest.raises(UserWarning) as e:
+            tables = camelot.read_pdf(filename, flavor="stream")
+        assert str(e.value) == "No tables found on page-1"
+
+
+def test_stream_no_tables_in_area():
+    filename = os.path.join(testdir, "only_page_number.pdf")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        with pytest.raises(UserWarning) as e:
+            tables = camelot.read_pdf(filename, flavor="stream")
+        assert str(e.value) == "No tables found in table area 1"
+
+
+def test_lattice_no_tables_on_page():
+    filename = os.path.join(testdir, "empty.pdf")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        with pytest.raises(UserWarning) as e:
+            tables = camelot.read_pdf(filename, flavor="lattice")
+        assert str(e.value) == "No tables found on page-1"
+
+
+def test_lattice_unknown_backend():
+    message = "Unknown backend 'mupdf' specified. Please use either 'poppler' or 'ghostscript'."
+    with pytest.raises(NotImplementedError, match=message):
+        tables = camelot.read_pdf(filename, backend="mupdf")
+
+
+def test_lattice_no_convert_method():
+    class ConversionBackend(object):
+        pass
+
+    message = "must implement a 'convert' method"
+    with pytest.raises(NotImplementedError, match=message):
+        tables = camelot.read_pdf(filename, backend=ConversionBackend())

tests/test_image_conversion_backend.py

@@ -4,18 +4,16 @@ import pytest
 
 import camelot.backends.image_conversion
 from camelot.backends import ImageConversionBackend
-from camelot.backends.poppler_backend import PopplerBackend
-from camelot.backends.ghostscript_backend import  GhostscriptBackend
 
 
 class PopplerBackendError(object):
     def convert(self, pdf_path, png_path):
-        raise ValueError('conversion failed')
+        raise ValueError("Image conversion failed")
 
 
 class GhostscriptBackendError(object):
     def convert(self, pdf_path, png_path):
-        raise ValueError('conversion failed')
+        raise ValueError("Image conversion failed")
 
 
 class GhostscriptBackendNoError(object):
@@ -24,26 +22,39 @@ class GhostscriptBackendNoError(object):
 
 
 def test_poppler_backend_error_when_no_use_fallback(monkeypatch):
-    backends = {"poppler": PopplerBackendError, "ghostscript": GhostscriptBackendNoError}
+    BACKENDS = {
-    monkeypatch.setattr("camelot.backends.image_conversion.backends", backends, raising=True)
+        "poppler": PopplerBackendError,
+        "ghostscript": GhostscriptBackendNoError,
+    }
+    monkeypatch.setattr(
+        "camelot.backends.image_conversion.BACKENDS", BACKENDS, raising=True
+    )
     backend = ImageConversionBackend(use_fallback=False)
 
-    message = "conversion failed with image conversion backend 'poppler'"
+    message = "Image conversion failed with image conversion backend 'poppler'"
     with pytest.raises(ValueError, match=message):
-        backend.convert('foo', 'bar')
+        backend.convert("foo", "bar")
+
 
 def test_ghostscript_backend_when_use_fallback(monkeypatch):
-    backends = {"poppler": PopplerBackendError, "ghostscript": GhostscriptBackendNoError}
+    BACKENDS = {
-    monkeypatch.setattr("camelot.backends.image_conversion.backends", backends, raising=True)
+        "poppler": PopplerBackendError,
+        "ghostscript": GhostscriptBackendNoError,
+    }
+    monkeypatch.setattr(
+        "camelot.backends.image_conversion.BACKENDS", BACKENDS, raising=True
+    )
     backend = ImageConversionBackend()
-    backend.convert('foo', 'bar')
+    backend.convert("foo", "bar")
 
 
 def test_ghostscript_backend_error_when_use_fallback(monkeypatch):
-    backends = {"poppler": PopplerBackendError, "ghostscript": GhostscriptBackendError}
+    BACKENDS = {"poppler": PopplerBackendError, "ghostscript": GhostscriptBackendError}
-    monkeypatch.setattr("camelot.backends.image_conversion.backends", backends, raising=True)
+    monkeypatch.setattr(
+        "camelot.backends.image_conversion.BACKENDS", BACKENDS, raising=True
+    )
     backend = ImageConversionBackend()
 
-    message = "conversion failed with image conversion backend 'ghostscript'"
+    message = "Image conversion failed with image conversion backend 'ghostscript'"
     with pytest.raises(ValueError, match=message):
-        backend.convert('foo', 'bar')
+        backend.convert("foo", "bar")

tests/test_plotting.py

@@ -6,9 +6,6 @@ import sys
 import pytest
 
 import camelot
-from camelot.backends.poppler_backend import PopplerBackend
-from camelot.backends.ghostscript_backend import GhostscriptBackend
-
 
 testdir = os.path.dirname(os.path.abspath(__file__))
 testdir = os.path.join(testdir, "files")
@@ -31,7 +28,7 @@ def test_textedge_plot():
 @pytest.mark.mpl_image_compare(baseline_dir="files/baseline_plots", remove_text=True)
 def test_lattice_contour_plot_poppler():
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=PopplerBackend())
+    tables = camelot.read_pdf(filename, backend="poppler")
     return camelot.plot(tables[0], kind="contour")
 
 
@@ -41,7 +38,7 @@ def test_lattice_contour_plot_ghostscript():
         pytest.skip("Skipping ghostscript test on Windows")
 
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=GhostscriptBackend())
+    tables = camelot.read_pdf(filename, backend="ghostscript")
     return camelot.plot(tables[0], kind="contour")
 
 
@@ -55,7 +52,7 @@ def test_stream_contour_plot():
 @pytest.mark.mpl_image_compare(baseline_dir="files/baseline_plots", remove_text=True)
 def test_line_plot_poppler():
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=PopplerBackend())
+    tables = camelot.read_pdf(filename, backend="poppler")
     return camelot.plot(tables[0], kind="line")
 
 
@@ -65,14 +62,14 @@ def test_line_plot_ghostscript():
         pytest.skip("Skipping ghostscript test on Windows")
 
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=GhostscriptBackend())
+    tables = camelot.read_pdf(filename, backend="ghostscript")
     return camelot.plot(tables[0], kind="line")
 
 
 @pytest.mark.mpl_image_compare(baseline_dir="files/baseline_plots", remove_text=True)
 def test_joint_plot_poppler():
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=PopplerBackend())
+    tables = camelot.read_pdf(filename, backend="poppler")
     return camelot.plot(tables[0], kind="joint")
 
 
@@ -82,14 +79,14 @@ def test_joint_plot_ghostscript():
         pytest.skip("Skipping ghostscript test on Windows")
 
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=GhostscriptBackend())
+    tables = camelot.read_pdf(filename, backend="ghostscript")
     return camelot.plot(tables[0], kind="joint")
 
 
 @pytest.mark.mpl_image_compare(baseline_dir="files/baseline_plots", remove_text=True)
 def test_grid_plot_poppler():
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=PopplerBackend())
+    tables = camelot.read_pdf(filename, backend="poppler")
     return camelot.plot(tables[0], kind="grid")
 
 
@@ -99,5 +96,5 @@ def test_grid_plot_ghostscript():
         pytest.skip("Skipping ghostscript test on Windows")
 
     filename = os.path.join(testdir, "foo.pdf")
-    tables = camelot.read_pdf(filename, backend=GhostscriptBackend())
+    tables = camelot.read_pdf(filename, backend="ghostscript")
     return camelot.plot(tables[0], kind="grid")