Adding pystache to third_party

third_party/pystache/src/context.py

Index: third_party/pystache/src/context.py
diff --git a/third_party/pystache/src/context.py b/third_party/pystache/src/context.py
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+++ b/third_party/pystache/src/context.py
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+# coding: utf-8
+
+"""
+Exposes a ContextStack class.
+
+The Mustache spec makes a special distinction between two types of context
+stack elements: hashes and objects.  For the purposes of interpreting the
+spec, we define these categories mutually exclusively as follows:
+
+ (1) Hash: an item whose type is a subclass of dict.
+
+ (2) Object: an item that is neither a hash nor an instance of a
+     built-in type.
+
+"""
+
+from pystache.common import PystacheError
+
+
+# This equals '__builtin__' in Python 2 and 'builtins' in Python 3.
+_BUILTIN_MODULE = type(0).__module__
+
+
+# We use this private global variable as a return value to represent a key
+# not being found on lookup.  This lets us distinguish between the case
+# of a key's value being None with the case of a key not being found --
+# without having to rely on exceptions (e.g. KeyError) for flow control.
+#
+# TODO: eliminate the need for a private global variable, e.g. by using the
+#   preferred Python approach of "easier to ask for forgiveness than permission":
+#     http://docs.python.org/glossary.html#term-eafp
+class NotFound(object):
+    pass
+_NOT_FOUND = NotFound()
+
+
+def _get_value(context, key):
+    """
+    Retrieve a key's value from a context item.
+
+    Returns _NOT_FOUND if the key does not exist.
+
+    The ContextStack.get() docstring documents this function's intended behavior.
+
+    """
+    if isinstance(context, dict):
+        # Then we consider the argument a "hash" for the purposes of the spec.
+        #
+        # We do a membership test to avoid using exceptions for flow control
+        # (e.g. catching KeyError).
+        if key in context:
+            return context[key]
+    elif type(context).__module__ != _BUILTIN_MODULE:
+        # Then we consider the argument an "object" for the purposes of
+        # the spec.
+        #
+        # The elif test above lets us avoid treating instances of built-in
+        # types like integers and strings as objects (cf. issue #81).
+        # Instances of user-defined classes on the other hand, for example,
+        # are considered objects by the test above.
+        try:
+            attr = getattr(context, key)
+        except AttributeError:
+            # TODO: distinguish the case of the attribute not existing from
+            #   an AttributeError being raised by the call to the attribute.
+            #   See the following issue for implementation ideas:
+            #     http://bugs.python.org/issue7559
+            pass
+        else:
+            # TODO: consider using EAFP here instead.
+            #   http://docs.python.org/glossary.html#term-eafp
+            if callable(attr):
+                return attr()
+            return attr
+
+    return _NOT_FOUND
+
+
+class KeyNotFoundError(PystacheError):
+
+    """
+    An exception raised when a key is not found in a context stack.
+
+    """
+
+    def __init__(self, key, details):
+        self.key = key
+        self.details = details
+
+    def __str__(self):
+        return "Key %s not found: %s" % (repr(self.key), self.details)
+
+
+class ContextStack(object):
+
+    """
+    Provides dictionary-like access to a stack of zero or more items.
+
+    Instances of this class are meant to act as the rendering context
+    when rendering Mustache templates in accordance with mustache(5)
+    and the Mustache spec.
+
+    Instances encapsulate a private stack of hashes, objects, and built-in
+    type instances.  Querying the stack for the value of a key queries
+    the items in the stack in order from last-added objects to first
+    (last in, first out).
+
+    Caution: this class does not currently support recursive nesting in
+    that items in the stack cannot themselves be ContextStack instances.
+
+    See the docstrings of the methods of this class for more details.
+
+    """
+
+    # We reserve keyword arguments for future options (e.g. a "strict=True"
+    # option for enabling a strict mode).
+    def __init__(self, *items):
+        """
+        Construct an instance, and initialize the private stack.
+
+        The *items arguments are the items with which to populate the
+        initial stack.  Items in the argument list are added to the
+        stack in order so that, in particular, items at the end of
+        the argument list are queried first when querying the stack.
+
+        Caution: items should not themselves be ContextStack instances, as
+        recursive nesting does not behave as one might expect.
+
+        """
+        self._stack = list(items)
+
+    def __repr__(self):
+        """
+        Return a string representation of the instance.
+
+        For example--
+
+        >>> context = ContextStack({'alpha': 'abc'}, {'numeric': 123})
+        >>> repr(context)
+        "ContextStack({'alpha': 'abc'}, {'numeric': 123})"
+
+        """
+        return "%s%s" % (self.__class__.__name__, tuple(self._stack))
+
+    @staticmethod
+    def create(*context, **kwargs):
+        """
+        Build a ContextStack instance from a sequence of context-like items.
+
+        This factory-style method is more general than the ContextStack class's
+        constructor in that, unlike the constructor, the argument list
+        can itself contain ContextStack instances.
+
+        Here is an example illustrating various aspects of this method:
+
+        >>> obj1 = {'animal': 'cat', 'vegetable': 'carrot', 'mineral': 'copper'}
+        >>> obj2 = ContextStack({'vegetable': 'spinach', 'mineral': 'silver'})
+        >>>
+        >>> context = ContextStack.create(obj1, None, obj2, mineral='gold')
+        >>>
+        >>> context.get('animal')
+        'cat'
+        >>> context.get('vegetable')
+        'spinach'
+        >>> context.get('mineral')
+        'gold'
+
+        Arguments:
+
+          *context: zero or more dictionaries, ContextStack instances, or objects
+            with which to populate the initial context stack.  None
+            arguments will be skipped.  Items in the *context list are
+            added to the stack in order so that later items in the argument
+            list take precedence over earlier items.  This behavior is the
+            same as the constructor's.
+
+          **kwargs: additional key-value data to add to the context stack.
+            As these arguments appear after all items in the *context list,
+            in the case of key conflicts these values take precedence over
+            all items in the *context list.  This behavior is the same as
+            the constructor's.
+
+        """
+        items = context
+
+        context = ContextStack()
+
+        for item in items:
+            if item is None:
+                continue
+            if isinstance(item, ContextStack):
+                context._stack.extend(item._stack)
+            else:
+                context.push(item)
+
+        if kwargs:
+            context.push(kwargs)
+
+        return context
+
+    # TODO: add more unit tests for this.
+    # TODO: update the docstring for dotted names.
+    def get(self, name):
+        """
+        Resolve a dotted name against the current context stack.
+
+        This function follows the rules outlined in the section of the
+        spec regarding tag interpolation.  This function returns the value
+        as is and does not coerce the return value to a string.
+
+        Arguments:
+
+          name: a dotted or non-dotted name.
+
+          default: the value to return if name resolution fails at any point.
+            Defaults to the empty string per the Mustache spec.
+
+        This method queries items in the stack in order from last-added
+        objects to first (last in, first out).  The value returned is
+        the value of the key in the first item that contains the key.
+        If the key is not found in any item in the stack, then the default
+        value is returned.  The default value defaults to None.
+
+        In accordance with the spec, this method queries items in the
+        stack for a key differently depending on whether the item is a
+        hash, object, or neither (as defined in the module docstring):
+
+        (1) Hash: if the item is a hash, then the key's value is the
+            dictionary value of the key.  If the dictionary doesn't contain
+            the key, then the key is considered not found.
+
+        (2) Object: if the item is an an object, then the method looks for
+            an attribute with the same name as the key.  If an attribute
+            with that name exists, the value of the attribute is returned.
+            If the attribute is callable, however (i.e. if the attribute
+            is a method), then the attribute is called with no arguments
+            and that value is returned.  If there is no attribute with
+            the same name as the key, then the key is considered not found.
+
+        (3) Neither: if the item is neither a hash nor an object, then
+            the key is considered not found.
+
+        *Caution*:
+
+          Callables are handled differently depending on whether they are
+          dictionary values, as in (1) above, or attributes, as in (2).
+          The former are returned as-is, while the latter are first
+          called and that value returned.
+
+          Here is an example to illustrate:
+
+          >>> def greet():
+          ...     return "Hi Bob!"
+          >>>
+          >>> class Greeter(object):
+          ...     greet = None
+          >>>
+          >>> dct = {'greet': greet}
+          >>> obj = Greeter()
+          >>> obj.greet = greet
+          >>>
+          >>> dct['greet'] is obj.greet
+          True
+          >>> ContextStack(dct).get('greet')  #doctest: +ELLIPSIS
+          <function greet at 0x...>
+          >>> ContextStack(obj).get('greet')
+          'Hi Bob!'
+
+          TODO: explain the rationale for this difference in treatment.
+
+        """
+        if name == '.':
+            try:
+                return self.top()
+            except IndexError:
+                raise KeyNotFoundError(".", "empty context stack")
+
+        parts = name.split('.')
+
+        try:
+            result = self._get_simple(parts[0])
+        except KeyNotFoundError:
+            raise KeyNotFoundError(name, "first part")
+
+        for part in parts[1:]:
+            # The full context stack is not used to resolve the remaining parts.
+            # From the spec--
+            #
+            #   5) If any name parts were retained in step 1, each should be
+            #   resolved against a context stack containing only the result
+            #   from the former resolution.  If any part fails resolution, the
+            #   result should be considered falsey, and should interpolate as
+            #   the empty string.
+            #
+            # TODO: make sure we have a test case for the above point.
+            result = _get_value(result, part)
+            # TODO: consider using EAFP here instead.
+            #   http://docs.python.org/glossary.html#term-eafp
+            if result is _NOT_FOUND:
+                raise KeyNotFoundError(name, "missing %s" % repr(part))
+
+        return result
+
+    def _get_simple(self, name):
+        """
+        Query the stack for a non-dotted name.
+
+        """
+        for item in reversed(self._stack):
+            result = _get_value(item, name)
+            if result is not _NOT_FOUND:
+                return result
+
+        raise KeyNotFoundError(name, "part missing")
+
+    def push(self, item):
+        """
+        Push an item onto the stack.
+
+        """
+        self._stack.append(item)
+
+    def pop(self):
+        """
+        Pop an item off of the stack, and return it.
+
+        """
+        return self._stack.pop()
+
+    def top(self):
+        """
+        Return the item last added to the stack.
+
+        """
+        return self._stack[-1]
+
+    def copy(self):
+        """
+        Return a copy of this instance.
+
+        """
+        return ContextStack(*self._stack)