add function to format the assertion message

This function is used to prettify the assertion message when a matcher failed and return an assertion error.

tests/unit/test_matchers.py

@@ -157,3 +157,18 @@ def test_metchers():
     assert_matcher('port')
     assert_matcher('path')
     assert_matcher('query')
+
+
+def test_get_assertion_message():
+    assert matchers.get_assertion_message(None) == ""
+    assert matchers.get_assertion_message("") == ""
+
+
+def test_get_assertion_message_with_details():
+    assertion_msg = "q1=1 != q2=1"
+    expected = (
+        "--------------- DETAILS ---------------\n"
+        "{}\n"
+        "----------------------------------------\n".format(assertion_msg)
+    )
+    assert matchers.get_assertion_message(assertion_msg) == expected

vcr/matchers.py

@@ -99,3 +99,23 @@ def requests_match(r1, r2, matchers):
     matches = [(m(r1, r2), m) for m in matchers]
     _log_matches(r1, r2, matches)
     return all(m[0] for m in matches)
+
+
+def get_assertion_message(assertion_details, **format_options):
+    """
+    Get a detailed message about the failing matcher.
+    """
+    msg = ""
+    if assertion_details:
+        separator = format_options.get("separator", "-")
+        title = format_options.get("title", " DETAILS ")
+        nb_separator = format_options.get("nb_separator", 40)
+        first_title_line = (
+            separator * ((nb_separator - len(title)) // 2)
+            + title
+            + separator * ((nb_separator - len(title)) // 2)
+        )
+        msg += "{}\n{}\n{}\n".format(
+            first_title_line, str(assertion_details), separator * nb_separator
+        )
+    return msg