ENH: Allow adding PageRange objects (#948)

Now the following is possible:

    >>> from PyPDF2 import PageRange
    >>> a = PageRange("0:5")
    >>> b = PageRange("2:7")
    >>> a + b
    PageRange("0:7")

Closes #759
See #751
See #752

PyPDF2/pagerange.py

@@ -116,6 +116,23 @@ def __eq__(self, other: Any) -> bool:
             return False
         return self._slice == other._slice
 
+    def __add__(self, other: "PageRange") -> "PageRange":
+        if not isinstance(other, PageRange):
+            raise TypeError(f"Can't add PageRange and {type(other)}")
+        if self._slice.step is not None or other._slice.step is not None:
+            raise ValueError("Can't add PageRange with stride")
+        a = self._slice.start, self._slice.stop
+        b = other._slice.start, other._slice.stop
+
+        if a[0] > b[0]:
+            a, b = b, a
+
+        # Now a[0] is the smallest
+        if b[0] > a[1]:
+            # There is a gap between a and b.
+            raise ValueError("Can't add PageRanges with gap")
+        return PageRange(slice(a[0], max(a[1], b[1])))
+
 
 PAGE_RANGE_ALL = PageRange(":")  # The range of all pages.
 

tests/test_pagerange.py

@@ -78,3 +78,45 @@ def test_parse_filename_page_ranges_err():
     assert (
         exc.value.args[0] == "The first argument must be a filename, not a page range."
     )
+
+
+@pytest.mark.parametrize(
+    "a, b, expected",
+    [
+        (PageRange(slice(0, 5)), PageRange(slice(2, 10)), slice(0, 10)),
+        (PageRange(slice(0, 5)), PageRange(slice(2, 3)), slice(0, 5)),
+        (PageRange(slice(0, 5)), PageRange(slice(5, 10)), slice(0, 10)),
+    ],
+)
+def test_addition(a, b, expected):
+    pr1 = PageRange(a)
+    pr2 = PageRange(b)
+    assert pr1 + pr2 == PageRange(expected)
+    assert pr2 + pr1 == PageRange(expected)  # addition is commutative
+
+
+@pytest.mark.parametrize(
+    "a, b",
+    [
+        (PageRange(slice(0, 5)), PageRange(slice(7, 10))),
+        (PageRange(slice(7, 10)), PageRange(slice(0, 5))),
+    ],
+)
+def test_addition_gap(a: PageRange, b: PageRange):
+    with pytest.raises(ValueError) as exc:
+        a + b
+    assert exc.value.args[0] == "Can't add PageRanges with gap"
+
+
+def test_addition_non_page_range():
+    with pytest.raises(TypeError) as exc:
+        PageRange(slice(0, 5)) + "2:7"
+    assert exc.value.args[0] == "Can't add PageRange and <class 'str'>"
+
+
+def test_addition_stride():
+    a = PageRange(slice(0, 5, 2))
+    b = PageRange(slice(7, 9))
+    with pytest.raises(ValueError) as exc:
+        a + b
+    assert exc.value.args[0] == "Can't add PageRange with stride"