text: """Rewrite assertion AST to produce nice error messages"""
  
  import ast
  import errno
  import itertools
  import imp
  import marshal
  import os
  import re
  import struct
  import sys
  import types
  
  import py
  from _pytest.assertion import util
  
  
  # pytest caches rewritten pycs in __pycache__.
  if hasattr(imp, "get_tag"):
      PYTEST_TAG = imp.get_tag() + "-PYTEST"
  else:
      if hasattr(sys, "pypy_version_info"):
          impl = "pypy"
      elif sys.platform == "java":
          impl = "jython"
      else:
          impl = "cpython"
      ver = sys.version_info
      PYTEST_TAG = "%s-%s%s-PYTEST" % (impl, ver[0], ver[1])
      del ver, impl
  
  PYC_EXT = ".py" + (__debug__ and "c" or "o")
  PYC_TAIL = "." + PYTEST_TAG + PYC_EXT
  
  REWRITE_NEWLINES = sys.version_info[:2] != (2, 7) and sys.version_info < (3, 2)
  ASCII_IS_DEFAULT_ENCODING = sys.version_info[0] < 3
  
  class AssertionRewritingHook(object):
      """PEP302 Import hook which rewrites asserts."""
  
      def __init__(self):
          self.session = None
          self.modules = {}
          self._register_with_pkg_resources()
  
      def set_session(self, session):
          self.fnpats = session.config.getini("python_files")
          self.session = session
  
      def find_module(self, name, path=None):
          if self.session is None:
              return None
          sess = self.session
          state = sess.config._assertstate
          state.trace("find_module called for: %s" % name)
          names = name.rsplit(".", 1)
          lastname = names[-1]
          pth = None
          if path is not None:
              # Starting with Python 3.3, path is a _NamespacePath(), which
              # causes problems if not converted to list.
              path = list(path)
              if len(path) == 1:
                  pth = path[0]
          if pth is None:
              try:
                  fd, fn, desc = imp.find_module(lastname, path)
              except ImportError:
                  return None
              if fd is not None:
                  fd.close()
              tp = desc[2]
              if tp == imp.PY_COMPILED:
                  if hasattr(imp, "source_from_cache"):
                      fn = imp.source_from_cache(fn)
                  else:
                      fn = fn[:-1]
              elif tp != imp.PY_SOURCE:
                  # Don't know what this is.
                  return None
          else:
              fn = os.path.join(pth, name.rpartition(".")[2] + ".py")
          fn_pypath = py.path.local(fn)
          # Is this a test file?
          if not sess.isinitpath(fn):
              # We have to be very careful here because imports in this code can
              # trigger a cycle.
              self.session = None
              try:
                  for pat in self.fnpats:
                      if fn_pypath.fnmatch(pat):
                          state.trace("matched test file %r" % (fn,))
                          break
                  else:
                      return None
              finally:
                  self.session = sess
          else:
              state.trace("matched test file (was specified on cmdline): %r" %
                          (fn,))
          # The requested module looks like a test file, so rewrite it. This is
          # the most magical part of the process: load the source, rewrite the
          # asserts, and load the rewritten source. We also cache the rewritten
          # module code in a special pyc. We must be aware of the possibility of
          # concurrent pytest processes rewriting and loading pycs. To avoid
          # tricky race conditions, we maintain the following invariant: The
          # cached pyc is always a complete, valid pyc. Operations on it must be
          # atomic. POSIX's atomic rename comes in handy.
          write = not sys.dont_write_bytecode
          cache_dir = os.path.join(fn_pypath.dirname, "__pycache__")
          if write:
              try:
                  os.mkdir(cache_dir)
              except OSError:
                  e = sys.exc_info()[1].errno
                  if e == errno.EEXIST:
                      # Either the __pycache__ directory already exists (the
                      # common case) or it's blocked by a non-dir node. In the
                      # latter case, we'll ignore it in _write_pyc.
                      pass
                  elif e in [errno.ENOENT, errno.ENOTDIR]:
                      # One of the path components was not a directory, likely
                      # because we're in a zip file.
                      write = False
                  elif e == errno.EACCES:
                      state.trace("read only directory: %r" % fn_pypath.dirname)
                      write = False
                  else:
                      raise
          cache_name = fn_pypath.basename[:-3] + PYC_TAIL
          pyc = os.path.join(cache_dir, cache_name)
          # Notice that even if we're in a read-only directory, I'm going
          # to check for a cached pyc. This may not be optimal...
          co = _read_pyc(fn_pypath, pyc, state.trace)
          if co is None:
              state.trace("rewriting %r" % (fn,))
              source_stat, co = _rewrite_test(state, fn_pypath)
              if co is None:
                  # Probably a SyntaxError in the test.
                  return None
              if write:
                  _make_rewritten_pyc(state, source_stat, pyc, co)
          else:
              state.trace("found cached rewritten pyc for %r" % (fn,))
          self.modules[name] = co, pyc
          return self
  
      def load_module(self, name):
          co, pyc = self.modules.pop(name)
          # I wish I could just call imp.load_compiled here, but __file__ has to
          # be set properly. In Python 3.2+, this all would be handled correctly
          # by load_compiled.
          mod = sys.modules[name] = imp.new_module(name)
          try:
              mod.__file__ = co.co_filename
              # Normally, this attribute is 3.2+.
              mod.__cached__ = pyc
              mod.__loader__ = self
              py.builtin.exec_(co, mod.__dict__)
          except:
              del sys.modules[name]
              raise
          return sys.modules[name]
  
  
  
      def is_package(self, name):
          try:
              fd, fn, desc = imp.find_module(name)
          except ImportError:
              return False
          if fd is not None:
              fd.close()
          tp = desc[2]
          return tp == imp.PKG_DIRECTORY
  
      @classmethod
      def _register_with_pkg_resources(cls):
          """
          Ensure package resources can be loaded from this loader. May be called
          multiple times, as the operation is idempotent.
          """
          try:
              import pkg_resources
              # access an attribute in case a deferred importer is present
              pkg_resources.__name__
          except ImportError:
              return
  
          # Since pytest tests are always located in the file system, the
          #  DefaultProvider is appropriate.
          pkg_resources.register_loader_type(cls, pkg_resources.DefaultProvider)
  
  
  def _write_pyc(state, co, source_stat, pyc):
      # Technically, we don't have to have the same pyc format as
      # (C)Python, since these "pycs" should never be seen by builtin
      # import. However, there's little reason deviate, and I hope
      # sometime to be able to use imp.load_compiled to load them. (See
      # the comment in load_module above.)
      try:
          fp = open(pyc, "wb")
      except IOError:
          err = sys.exc_info()[1].errno
          state.trace("error writing pyc file at %s: errno=%s" %(pyc, err))
          # we ignore any failure to write the cache file
          # there are many reasons, permission-denied, __pycache__ being a
          # file etc.
          return False
      try:
          fp.write(imp.get_magic())
          mtime = int(source_stat.mtime)
          size = source_stat.size & 0xFFFFFFFF
          fp.write(struct.pack("<ll", mtime, size))
          marshal.dump(co, fp)
      finally:
          fp.close()
      return True
  
  RN = "\r\n".encode("utf-8")
  N = "\n".encode("utf-8")
  
  cookie_re = re.compile(r"^[ \t\f]*#.*coding[:=][ \t]*[-\w.]+")
  BOM_UTF8 = '\xef\xbb\xbf'
  
  def _rewrite_test(state, fn):
      """Try to read and rewrite *fn* and return the code object."""
      try:
          stat = fn.stat()
          source = fn.read("rb")
      except EnvironmentError:
          return None, None
      if ASCII_IS_DEFAULT_ENCODING:
          # ASCII is the default encoding in Python 2. Without a coding
          # declaration, Python 2 will complain about any bytes in the file
          # outside the ASCII range. Sadly, this behavior does not extend to
          # compile() or ast.parse(), which prefer to interpret the bytes as
          # latin-1. (At least they properly handle explicit coding cookies.) To
          # preserve this error behavior, we could force ast.parse() to use ASCII
          # as the encoding by inserting a coding cookie. Unfortunately, that
          # messes up line numbers. Thus, we have to check ourselves if anything
          # is outside the ASCII range in the case no encoding is explicitly
          # declared. For more context, see issue #269. Yay for Python 3 which
          # gets this right.
          end1 = source.find("\n")
          end2 = source.find("\n", end1 + 1)
          if (not source.startswith(BOM_UTF8) and
              cookie_re.match(source[0:end1]) is None and
              cookie_re.match(source[end1 + 1:end2]) is None):
              if hasattr(state, "_indecode"):
                  # encodings imported us again, so don't rewrite.
                  return None, None
              state._indecode = True
              try:
                  try:
                      source.decode("ascii")
                  except UnicodeDecodeError:
                      # Let it fail in real import.
                      return None, None
              finally:
                  del state._indecode
      # On Python versions which are not 2.7 and less than or equal to 3.1, the
      # parser expects *nix newlines.
      if REWRITE_NEWLINES:
          source = source.replace(RN, N) + N
      try:
          tree = ast.parse(source)
      except SyntaxError:
          # Let this pop up again in the real import.
          state.trace("failed to parse: %r" % (fn,))
          return None, None
      rewrite_asserts(tree)
      try:
          co = compile(tree, fn.strpath, "exec")
      except SyntaxError:
          # It's possible that this error is from some bug in the
          # assertion rewriting, but I don't know of a fast way to tell.
          state.trace("failed to compile: %r" % (fn,))
          return None, None
      return stat, co
  
  def _make_rewritten_pyc(state, source_stat, pyc, co):
      """Try to dump rewritten code to *pyc*."""
      if sys.platform.startswith("win"):
          # Windows grants exclusive access to open files and doesn't have atomic
          # rename, so just write into the final file.
          _write_pyc(state, co, source_stat, pyc)
      else:
          # When not on windows, assume rename is atomic. Dump the code object
          # into a file specific to this process and atomically replace it.
          proc_pyc = pyc + "." + str(os.getpid())
          if _write_pyc(state, co, source_stat, proc_pyc):
              os.rename(proc_pyc, pyc)
  
  def _read_pyc(source, pyc, trace=lambda x: None):
      """Possibly read a pytest pyc containing rewritten code.
  
      Return rewritten code if successful or None if not.
      """
      try:
          fp = open(pyc, "rb")
      except IOError:
          return None
      with fp:
          try:
              mtime = int(source.mtime())
              size = source.size()
              data = fp.read(12)
          except EnvironmentError as e:
              trace('_read_pyc(%s): EnvironmentError %s' % (source, e))
              return None
          # Check for invalid or out of date pyc file.
          if (len(data) != 12 or data[:4] != imp.get_magic() or
                  struct.unpack("<ll", data[4:]) != (mtime, size)):
              trace('_read_pyc(%s): invalid or out of date pyc' % source)
              return None
          try:
              co = marshal.load(fp)
          except Exception as e:
              trace('_read_pyc(%s): marshal.load error %s' % (source, e))
              return None
          if not isinstance(co, types.CodeType):
              trace('_read_pyc(%s): not a code object' % source)
              return None
          return co
  
  
  def rewrite_asserts(mod):
      """Rewrite the assert statements in mod."""
      AssertionRewriter().run(mod)
  
  
  def _saferepr(obj):
      """Get a safe repr of an object for assertion error messages.
  
      The assertion formatting (util.format_explanation()) requires
      newlines to be escaped since they are a special character for it.
      Normally assertion.util.format_explanation() does this but for a
      custom repr it is possible to contain one of the special escape
      sequences, especially '\n{' and '\n}' are likely to be present in
      JSON reprs.
  
      """
      repr = py.io.saferepr(obj)
      if py.builtin._istext(repr):
          t = py.builtin.text
      else:
          t = py.builtin.bytes
      return repr.replace(t("\n"), t("\\n"))
  
  
  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~")).replace(t("%"), t("%%"))
      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)
  
  def _format_boolop(explanations, is_or):
      return "(" + (is_or and " or " or " and ").join(explanations) + ")"
  
  def _call_reprcompare(ops, results, expls, each_obj):
      for i, res, expl in zip(range(len(ops)), results, expls):
          try:
              done = not res
          except Exception:
              done = True
          if done:
              break
      if util._reprcompare is not None:
          custom = util._reprcompare(ops[i], each_obj[i], each_obj[i + 1])
          if custom is not None:
              return custom
      return expl
  
  
  unary_map = {
      ast.Not: "not %s",
      ast.Invert: "~%s",
      ast.USub: "-%s",
      ast.UAdd: "+%s"
  }
  
  binop_map = {
      ast.BitOr: "|",
      ast.BitXor: "^",
      ast.BitAnd: "&",
      ast.LShift: "<<",
      ast.RShift: ">>",
      ast.Add: "+",
      ast.Sub: "-",
      ast.Mult: "*",
      ast.Div: "/",
      ast.FloorDiv: "//",
      ast.Mod: "%%", # escaped for string formatting
      ast.Eq: "==",
      ast.NotEq: "!=",
      ast.Lt: "<",
      ast.LtE: "<=",
      ast.Gt: ">",
      ast.GtE: ">=",
      ast.Pow: "**",
      ast.Is: "is",
      ast.IsNot: "is not",
      ast.In: "in",
      ast.NotIn: "not in"
  }
  
  
  def set_location(node, lineno, col_offset):
      """Set node location information recursively."""
      def _fix(node, lineno, col_offset):
          if "lineno" in node._attributes:
              node.lineno = lineno
          if "col_offset" in node._attributes:
              node.col_offset = col_offset
          for child in ast.iter_child_nodes(node):
              _fix(child, lineno, col_offset)
      _fix(node, lineno, col_offset)
      return node
  
  
  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."""
          if not mod.body:
              # Nothing to do.
              return
          # Insert some special imports at the top of the module but after any
          # docstrings and __future__ imports.
          aliases = [ast.alias(py.builtin.builtins.__name__, "@py_builtins"),
                     ast.alias("_pytest.assertion.rewrite", "@pytest_ar")]
          expect_docstring = True
          pos = 0
          lineno = 0
          for item in mod.body:
              if (expect_docstring and isinstance(item, ast.Expr) and
                      isinstance(item.value, ast.Str)):
                  doc = item.value.s
                  if "PYTEST_DONT_REWRITE" in doc:
                      # The module has disabled assertion rewriting.
                      return
                  lineno += len(doc) - 1
                  expect_docstring = False
              elif (not isinstance(item, ast.ImportFrom) or item.level > 0 or
                    item.module != "__future__"):
                  lineno = item.lineno
                  break
              pos += 1
          imports = [ast.Import([alias], lineno=lineno, col_offset=0)
                     for alias in aliases]
          mod.body[pos:pos] = imports
          # Collect asserts.
          nodes = [mod]
          while nodes:
              node = nodes.pop()
              for name, field in ast.iter_fields(node):
                  if isinstance(field, list):
                      new = []
                      for i, child in enumerate(field):
                          if isinstance(child, ast.Assert):
                              # Transform assert.
                              new.extend(self.visit(child))
                          else:
                              new.append(child)
                              if isinstance(child, ast.AST):
                                  nodes.append(child)
                      setattr(node, name, new)
                  elif (isinstance(field, ast.AST) and
                        # Don't recurse into expressions as they can't contain
                        # asserts.
                        not isinstance(field, ast.expr)):
                      nodes.append(field)
  
      def variable(self):
          """Get a new variable."""
          # Use a character invalid in python identifiers to avoid clashing.
          name = "@py_assert" + str(next(self.variable_counter))
          self.variables.append(name)
          return name
  
      def assign(self, expr):
          """Give *expr* a name."""
          name = self.variable()
          self.statements.append(ast.Assign([ast.Name(name, ast.Store())], expr))
          return ast.Name(name, ast.Load())
  
      def display(self, expr):
          """Call py.io.saferepr on the expression."""
          return self.helper("saferepr", expr)
  
      def helper(self, name, *args):
          """Call a helper in this module."""
          py_name = ast.Name("@pytest_ar", ast.Load())
          attr = ast.Attribute(py_name, "_" + name, ast.Load())
          return ast.Call(attr, list(args), [], None, None)
  
      def builtin(self, name):
          """Return the builtin called *name*."""
          builtin_name = ast.Name("@py_builtins", ast.Load())
          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]
          keys = [ast.Str(key) for key in current.keys()]
          format_dict = ast.Dict(keys, list(current.values()))
          form = ast.BinOp(expl_expr, ast.Mod(), format_dict)
          name = "@py_format" + str(next(self.variable_counter))
          self.on_failure.append(ast.Assign([ast.Name(name, ast.Store())], form))
          return ast.Name(name, ast.Load())
  
      def generic_visit(self, node):
          """Handle expressions we don't have custom code for."""
          assert isinstance(node, ast.expr)
          res = self.assign(node)
          return res, self.explanation_param(self.display(res))
  
      def visit_Assert(self, 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.variables = []
          self.variable_counter = itertools.count()
          self.stack = []
          self.on_failure = []
          self.push_format_context()
          # Rewrite assert into a bunch of statements.
          top_condition, explanation = self.visit(assert_.test)
          # Create failure message.
          body = self.on_failure
          negation = ast.UnaryOp(ast.Not(), top_condition)
          self.statements.append(ast.If(negation, body, []))
          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())
          exc = ast.Call(err_name, [fmt], [], None, None)
          if sys.version_info[0] >= 3:
              raise_ = ast.Raise(exc, None)
          else:
              raise_ = ast.Raise(exc, None, None)
          body.append(raise_)
          # Clear temporary variables by setting them to None.
          if self.variables:
              variables = [ast.Name(name, ast.Store())
                           for name in self.variables]
              clear = ast.Assign(variables, ast.Name("None", ast.Load()))
              self.statements.append(clear)
          # Fix line numbers.
          for stmt in self.statements:
              set_location(stmt, assert_.lineno, assert_.col_offset)
          return self.statements
  
      def visit_Name(self, name):
          # Display the repr of the name if it's a local variable or
          # _should_repr_global_name() thinks it's acceptable.
          locs = ast.Call(self.builtin("locals"), [], [], None, None)
          inlocs = ast.Compare(ast.Str(name.id), [ast.In()], [locs])
          dorepr = self.helper("should_repr_global_name", name)
          test = ast.BoolOp(ast.Or(), [inlocs, dorepr])
          expr = ast.IfExp(test, self.display(name), ast.Str(name.id))
          return name, self.explanation_param(expr)
  
      def visit_BoolOp(self, boolop):
          res_var = self.variable()
          expl_list = self.assign(ast.List([], ast.Load()))
          app = ast.Attribute(expl_list, "append", ast.Load())
          is_or = int(isinstance(boolop.op, ast.Or))
          body = save = self.statements
          fail_save = self.on_failure
          levels = len(boolop.values) - 1
          self.push_format_context()
          # Process each operand, short-circuting if needed.
          for i, v in enumerate(boolop.values):
              if i:
                  fail_inner = []
                  # cond is set in a prior loop iteration below
                  self.on_failure.append(ast.If(cond, fail_inner, [])) # noqa
                  self.on_failure = fail_inner
              self.push_format_context()
              res, expl = self.visit(v)
              body.append(ast.Assign([ast.Name(res_var, ast.Store())], res))
              expl_format = self.pop_format_context(ast.Str(expl))
              call = ast.Call(app, [expl_format], [], None, None)
              self.on_failure.append(ast.Expr(call))
              if i < levels:
                  cond = res
                  if is_or:
                      cond = ast.UnaryOp(ast.Not(), cond)
                  inner = []
                  self.statements.append(ast.If(cond, inner, []))
                  self.statements = body = inner
          self.statements = save
          self.on_failure = fail_save
          expl_template = self.helper("format_boolop", expl_list, ast.Num(is_or))
          expl = self.pop_format_context(expl_template)
          return ast.Name(res_var, ast.Load()), self.explanation_param(expl)
  
      def visit_UnaryOp(self, unary):
          pattern = unary_map[unary.op.__class__]
          operand_res, operand_expl = self.visit(unary.operand)
          res = self.assign(ast.UnaryOp(unary.op, operand_res))
          return res, pattern % (operand_expl,)
  
      def visit_BinOp(self, binop):
          symbol = binop_map[binop.op.__class__]
          left_expr, left_expl = self.visit(binop.left)
          right_expr, right_expl = self.visit(binop.right)
          explanation = "(%s %s %s)" % (left_expl, symbol, right_expl)
          res = self.assign(ast.BinOp(left_expr, binop.op, right_expr))
          return res, explanation
  
      def visit_Call(self, call):
          new_func, func_expl = self.visit(call.func)
          arg_expls = []
          new_args = []
          new_kwargs = []
          new_star = new_kwarg = None
          for arg in call.args:
              res, expl = self.visit(arg)
              new_args.append(res)
              arg_expls.append(expl)
          for keyword in call.keywords:
              res, expl = self.visit(keyword.value)
              new_kwargs.append(ast.keyword(keyword.arg, res))
              arg_expls.append(keyword.arg + "=" + expl)
          if call.starargs:
              new_star, expl = self.visit(call.starargs)
              arg_expls.append("*" + expl)
          if call.kwargs:
              new_kwarg, expl = self.visit(call.kwargs)
              arg_expls.append("**" + expl)
          expl = "%s(%s)" % (func_expl, ', '.join(arg_expls))
          new_call = ast.Call(new_func, new_args, new_kwargs,
                              new_star, new_kwarg)
          res = self.assign(new_call)
          res_expl = self.explanation_param(self.display(res))
          outer_expl = "%s\n{%s = %s\n}" % (res_expl, res_expl, expl)
          return res, outer_expl
  
      def visit_Attribute(self, attr):
          if not isinstance(attr.ctx, ast.Load):
              return self.generic_visit(attr)
          value, value_expl = self.visit(attr.value)
          res = self.assign(ast.Attribute(value, attr.attr, ast.Load()))
          res_expl = self.explanation_param(self.display(res))
          pat = "%s\n{%s = %s.%s\n}"
          expl = pat % (res_expl, res_expl, value_expl, attr.attr)
          return res, expl
  
      def visit_Compare(self, comp):
          self.push_format_context()
          left_res, left_expl = self.visit(comp.left)
          res_variables = [self.variable() for i in range(len(comp.ops))]
          load_names = [ast.Name(v, ast.Load()) for v in res_variables]
          store_names = [ast.Name(v, ast.Store()) for v in res_variables]
          it = zip(range(len(comp.ops)), comp.ops, comp.comparators)
          expls = []
          syms = []
          results = [left_res]
          for i, op, next_operand in it:
              next_res, next_expl = self.visit(next_operand)
              results.append(next_res)
              sym = binop_map[op.__class__]
              syms.append(ast.Str(sym))
              expl = "%s %s %s" % (left_expl, sym, next_expl)
              expls.append(ast.Str(expl))
              res_expr = ast.Compare(left_res, [op], [next_res])
              self.statements.append(ast.Assign([store_names[i]], res_expr))
              left_res, left_expl = next_res, next_expl
          # Use pytest.assertion.util._reprcompare if that's available.
          expl_call = self.helper("call_reprcompare",
                                  ast.Tuple(syms, ast.Load()),
                                  ast.Tuple(load_names, ast.Load()),
                                  ast.Tuple(expls, ast.Load()),
                                  ast.Tuple(results, ast.Load()))
          if len(comp.ops) > 1:
              res = ast.BoolOp(ast.And(), load_names)
          else:
              res = load_names[0]
          return res, self.explanation_param(self.pop_format_context(expl_call))
  