Do not fail to parse rationals stored as '0/0'.

In the wonderful world of EXIF metadata, this means a null fraction, i.e. 0/1.

4 files changed, 58 insertions, 10 deletions

diff --git a/doc/api.rst b/doc/api.rst
index c48769d..a91122b 100644
--- a/doc/api.rst
+++ b/doc/api.rst
@@ -64,6 +64,7 @@ pyexiv2.utils
 .. module:: pyexiv2.utils
 .. autofunction:: undefined_to_string
 .. autofunction:: string_to_undefined
+.. autofunction:: make_fraction
 
 .. autoclass:: Rational
    :members: numerator, denominator, from_string, to_float, __eq__
diff --git a/src/pyexiv2/utils.py b/src/pyexiv2/utils.py
index 4fcebf9..22a05f7 100644
--- a/src/pyexiv2/utils.py
+++ b/src/pyexiv2/utils.py
@@ -170,6 +170,9 @@ class Rational(object):
     A class representing a rational number.
 
     Its numerator and denominator are read-only properties.
+
+    Do not use this class directly to instantiate a rational number.
+    Instead, use :func:`make_fraction`.
     """
 
     _format_re = re.compile(r'(?P<numerator>-?\d+)/(?P<denominator>\d+)')
@@ -200,6 +203,27 @@ class Rational(object):
         return self._denominator
 
     @staticmethod
+    def match_string(string):
+        """
+        Match a string against the expected format for a :class:`Rational`
+        (``[-]numerator/denominator``) and return the numerator and denominator
+        as a tuple.
+
+        :param string: a string representation of a rational number
+        :type string: string
+
+        :return: a tuple (numerator, denominator)
+        :rtype: tuple of long
+
+        :raise ValueError: if the format of the string is invalid
+        """
+        match = Rational._format_re.match(string)
+        if match is None:
+            raise ValueError('Invalid format for a rational: %s' % string)
+        gd = match.groupdict()
+        return (long(gd['numerator']), long(gd['denominator']))
+
+    @staticmethod
     def from_string(string):
         """
         Instantiate a :class:`Rational` from a string formatted as
@@ -213,11 +237,8 @@ class Rational(object):
 
         :raise ValueError: if the format of the string is invalid
         """
-        match = Rational._format_re.match(string)
-        if match is None:
-            raise ValueError('Invalid format for a rational: %s' % string)
-        gd = match.groupdict()
-        return Rational(long(gd['numerator']), long(gd['denominator']))
+        numerator, denominator = Rational.match_string(string)
+        return Rational(numerator, denominator)
 
     def to_float(self):
         """
@@ -267,14 +288,26 @@ def make_fraction(*args):
 
     The type of the returned object depends on the availability of the
     fractions module in the standard library (Python ≥ 2.6).
+
+    :raise TypeError: if the arguments do not match the expected format for a
+                      fraction
     """
+    if len(args) == 1:
+        numerator, denominator = Rational.match_string(args[0])
+    elif len(args) == 2:
+        numerator = args[0]
+        denominator = args[1]
+    else:
+        raise TypeError('Invalid format for a fraction: %s' % str(args))
+    if denominator == 0 and numerator == 0:
+        # Null rationals are often stored as '0/0'.
+        # We want to be fault-tolerant in this specific case
+        # (see https://bugs.launchpad.net/pyexiv2/+bug/786253).
+        denominator = 1
     if Fraction is not None:
-        return Fraction(*args)
+        return Fraction(numerator, denominator)
     else:
-        if len(args) == 1:
-            return Rational.from_string(*args)
-        else:
-            return Rational(*args)
+        return Rational(numerator, denominator)
 
 
 def fraction_to_string(fraction):
diff --git a/test/rational.py b/test/rational.py
index 1ee5670..bff8e5f 100644
--- a/test/rational.py
+++ b/test/rational.py
@@ -52,6 +52,16 @@ class TestRational(unittest.TestCase):
         else:
             self.fail('Denominator is not read-only.')
 
+    def test_match_string(self):
+        self.assertEqual(Rational.match_string('4/3'), (4, 3))
+        self.assertEqual(Rational.match_string('-4/3'), (-4, 3))
+        self.assertEqual(Rational.match_string('0/3'), (0, 3))
+        self.assertEqual(Rational.match_string('0/0'), (0, 0))
+        self.assertRaises(ValueError, Rational.match_string, '+3/5')
+        self.assertRaises(ValueError, Rational.match_string, '3 / 5')
+        self.assertRaises(ValueError, Rational.match_string, '3/-5')
+        self.assertRaises(ValueError, Rational.match_string, 'invalid')
+
     def test_from_string(self):
         self.assertEqual(Rational.from_string('4/3'), Rational(4, 3))
         self.assertEqual(Rational.from_string('-4/3'), Rational(-4, 3))
@@ -59,6 +69,8 @@ class TestRational(unittest.TestCase):
         self.assertRaises(ValueError, Rational.from_string, '3 / 5')
         self.assertRaises(ValueError, Rational.from_string, '3/-5')
         self.assertRaises(ValueError, Rational.from_string, 'invalid')
+        self.assertRaises(ZeroDivisionError, Rational.from_string, '1/0')
+        self.assertRaises(ZeroDivisionError, Rational.from_string, '0/0')
 
     def test_to_string(self):
         self.assertEqual(str(Rational(3, 5)), '3/5')
diff --git a/test/utils.py b/test/utils.py
index 11b1766..0b55254 100644
--- a/test/utils.py
+++ b/test/utils.py
@@ -78,11 +78,13 @@ class TestFractions(unittest.TestCase):
             self.assertEqual(make_fraction(-3, 5), Fraction(-3, 5))
             self.assertEqual(make_fraction('3/2'), Fraction(3, 2))
             self.assertEqual(make_fraction('-3/4'), Fraction(-3, 4))
+            self.assertEqual(make_fraction('0/0'), Fraction(0, 1))
         else:
             self.assertEqual(make_fraction(3, 5), Rational(3, 5))
             self.assertEqual(make_fraction(-3, 5), Rational(-3, 5))
             self.assertEqual(make_fraction('3/2'), Rational(3, 2))
             self.assertEqual(make_fraction('-3/4'), Rational(-3, 4))
+            self.assertEqual(make_fraction('0/0'), Rational(0, 1))
 
         self.assertRaises(ZeroDivisionError, make_fraction, 3, 0)
         self.assertRaises(ZeroDivisionError, make_fraction, '3/0')