Show user assertion messages and instrospection together

User provided messages, or any valid expression given as second
argument to the assert statement, are now shown in addition to the
py.test introspection details.  Formerly any user provided message
would entirely replace the introspection details.

Fixes issue549.

_pytest/assertion/rewrite.py

@@ -329,6 +329,33 @@ def rewrite_asserts(mod):
 _saferepr = py.io.saferepr
 from _pytest.assertion.util import format_explanation as _format_explanation # noqa
 
+def _format_assertmsg(obj):
+    """Format the custom assertion message given.
+
+    For strings this simply replaces newlines with '\n~' so that
+    util.format_explanation() will preserve them instead of escaping
+    newlines.  For other objects py.io.saferepr() is used first.
+
+    """
+    # reprlib appears to have a bug which means that if a string
+    # contains a newline it gets escaped, however if an object has a
+    # .__repr__() which contains newlines it does not get escaped.
+    # However in either case we want to preserve the newline.
+    if py.builtin._istext(obj) or py.builtin._isbytes(obj):
+        s = obj
+        is_repr = False
+    else:
+        s = py.io.saferepr(obj)
+        is_repr = True
+    if py.builtin._istext(s):
+        t = py.builtin.text
+    else:
+        t = py.builtin.bytes
+    s = s.replace(t("\n"), t("\n~"))
+    if is_repr:
+        s = s.replace(t("\\n"), t("\n~"))
+    return s
+
 def _should_repr_global_name(obj):
     return not hasattr(obj, "__name__") and not py.builtin.callable(obj)
 
@@ -397,6 +424,56 @@ def set_location(node, lineno, col_offset):
 
 
 class AssertionRewriter(ast.NodeVisitor):
+    """Assertion rewriting implementation.
+
+    The main entrypoint is to call .run() with an ast.Module instance,
+    this will then find all the assert statements and re-write them to
+    provide intermediate values and a detailed assertion error.  See
+    http://pybites.blogspot.be/2011/07/behind-scenes-of-pytests-new-assertion.html
+    for an overview of how this works.
+
+    The entry point here is .run() which will iterate over all the
+    statenemts in an ast.Module and for each ast.Assert statement it
+    finds call .visit() with it.  Then .visit_Assert() takes over and
+    is responsible for creating new ast statements to replace the
+    original assert statement: it re-writes the test of an assertion
+    to provide intermediate values and replace it with an if statement
+    which raises an assertion error with a detailed explanation in
+    case the expression is false.
+
+    For this .visit_Assert() uses the visitor pattern to visit all the
+    AST nodes of the ast.Assert.test field, each visit call returning
+    an AST node and the corresponding explanation string.  During this
+    state is kept in several instance attributes:
+
+    :statements: All the AST statements which will replace the assert
+       statement.
+
+    :variables: This is populated by .variable() with each variable
+       used by the statements so that they can all be set to None at
+       the end of the statements.
+
+    :variable_counter: Counter to create new unique variables needed
+       by statements.  Variables are created using .variable() and
+       have the form of "@py_assert0".
+
+    :on_failure: The AST statements which will be executed if the
+       assertion test fails.  This is the code which will construct
+       the failure message and raises the AssertionError.
+
+    :explanation_specifiers: A dict filled by .explanation_param()
+       with %-formatting placeholders and their corresponding
+       expressions to use in the building of an assertion message.
+       This is used by .pop_format_context() to build a message.
+
+    :stack: A stack of the explanation_specifiers dicts maintained by
+       .push_format_context() and .pop_format_context() which allows
+       to build another %-formatted string while already building one.
+
+    This state is reset on every new assert statement visited and used
+    by the other visitors.
+
+    """
 
     def run(self, mod):
         """Find all assert statements in *mod* and rewrite them."""
@@ -478,15 +555,41 @@ class AssertionRewriter(ast.NodeVisitor):
         return ast.Attribute(builtin_name, name, ast.Load())
 
     def explanation_param(self, expr):
+        """Return a new named %-formatting placeholder for expr.
+
+        This creates a %-formatting placeholder for expr in the
+        current formatting context, e.g. ``%(py0)s``.  The placeholder
+        and expr are placed in the current format context so that it
+        can be used on the next call to .pop_format_context().
+
+        """
         specifier = "py" + str(next(self.variable_counter))
         self.explanation_specifiers[specifier] = expr
         return "%(" + specifier + ")s"
 
     def push_format_context(self):
+        """Create a new formatting context.
+
+        The format context is used for when an explanation wants to
+        have a variable value formatted in the assertion message.  In
+        this case the value required can be added using
+        .explanation_param().  Finally .pop_format_context() is used
+        to format a string of %-formatted values as added by
+        .explanation_param().
+
+        """
         self.explanation_specifiers = {}
         self.stack.append(self.explanation_specifiers)
 
     def pop_format_context(self, expl_expr):
+        """Format the %-formatted string with current format context.
+
+        The expl_expr should be an ast.Str instance constructed from
+        the %-placeholders created by .explanation_param().  This will
+        add the required code to format said string to .on_failure and
+        return the ast.Name instance of the formatted string.
+
+        """
         current = self.stack.pop()
         if self.stack:
             self.explanation_specifiers = self.stack[-1]
@@ -504,11 +607,15 @@ class AssertionRewriter(ast.NodeVisitor):
         return res, self.explanation_param(self.display(res))
 
     def visit_Assert(self, assert_):
-        if assert_.msg:
-            # There's already a message. Don't mess with it.
-            return [assert_]
+        """Return the AST statements to replace the ast.Assert instance.
+
+        This re-writes the test of an assertion to provide
+        intermediate values and replace it with an if statement which
+        raises an assertion error with a detailed explanation in case
+        the expression is false.
+
+        """
         self.statements = []
-        self.cond_chain = ()
         self.variables = []
         self.variable_counter = itertools.count()
         self.stack = []
@@ -520,8 +627,13 @@ class AssertionRewriter(ast.NodeVisitor):
         body = self.on_failure
         negation = ast.UnaryOp(ast.Not(), top_condition)
         self.statements.append(ast.If(negation, body, []))
-        explanation = "assert " + explanation
-        template = ast.Str(explanation)
+        if assert_.msg:
+            assertmsg = self.helper('format_assertmsg', assert_.msg)
+            explanation = "\n>assert " + explanation
+        else:
+            assertmsg = ast.Str("")
+            explanation = "assert " + explanation
+        template = ast.BinOp(assertmsg, ast.Add(), ast.Str(explanation))
         msg = self.pop_format_context(template)
         fmt = self.helper("format_explanation", msg)
         err_name = ast.Name("AssertionError", ast.Load())