Fix Page.extract_table(...) when no table found

Return None instead of crashing.

Fixes https://github.com/jsvine/pdfplumber/issues/216

pdfplumber/page.py

@@ -174,6 +174,10 @@ class Page(Container):
 
     def extract_table(self, table_settings={}):
         tables = self.find_tables(table_settings)
+        
+        if len(tables) == 0:
+            return None
+
         # Return the largest table, as measured by number of cells.
         sorter = lambda x: (-len(x.cells), x.bbox[1], x.bbox[0])
         largest = list(sorted(tables, key=sorter))[0]

tests/test-issues.py

@@ -148,8 +148,17 @@ class Test(unittest.TestCase):
             cropped_page = page.crop((0, 0, page.width, 122))
             assert len(cropped_page.extract_table()) == 5
 
-
     def test_issue_203(self):
         path = os.path.join(HERE, "pdfs/issue-203-decimalize.pdf")
         with pdfplumber.open(path) as pdf:
             assert len(pdf.objects)
+
+    def test_issue_216(self):
+        """
+        .extract_table() should return None if there's no table,
+        instead of crashing
+        """
+        path = os.path.join(HERE, "pdfs/issue-140-example.pdf")
+        with pdfplumber.open(path) as pdf:
+            cropped = pdf.pages[0].crop((0, 0, 1, 1))
+            assert cropped.extract_table() is None