pystache.context module

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.

class pystache.context.ContextStack(*items)

Bases: 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.

copy()

Return a copy of this instance.

static 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.

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.

get(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')  
<function greet at 0x...>
>>> ContextStack(obj).get('greet')
'Hi Bob!'

TODO: explain the rationale for this difference in treatment.

pop()

Pop an item off of the stack, and return it.

push(item)

Push an item onto the stack.

top()

Return the item last added to the stack.

exception pystache.context.KeyNotFoundError(key, details)

Bases: PystacheError

An exception raised when a key is not found in a context stack.

class pystache.context.NotFound

Bases: object