class nltk.sem.Assignment(domain, assign=None)[source]

A dictionary which represents an assignment of values to variables.

An assigment can only assign values from its domain.

If an unknown expression a is passed to a model M‘s interpretation function i, i will first check whether M‘s valuation assigns an interpretation to a as a constant, and if this fails, i will delegate the interpretation of a to g. g only assigns values to individual variables (i.e., members of the class IndividualVariableExpression in the logic module. If a variable is not assigned a value by g, it will raise an Undefined exception.

A variable Assignment is a mapping from individual variables to entities in the domain. Individual variables are usually indicated with the letters 'x', 'y', 'w' and 'z', optionally followed by an integer (e.g., 'x0', 'y332'). Assignments are created using the Assignment constructor, which also takes the domain as a parameter.

>>> from nltk.sem.evaluate import Assignment
>>> dom = set(['u1', 'u2', 'u3', 'u4'])
>>> g3 = Assignment(dom, [('x', 'u1'), ('y', 'u2')])
>>> g3 == {'x': 'u1', 'y': 'u2'}

There is also a print format for assignments which uses a notation closer to that in logic textbooks:

>>> print(g3)

It is also possible to update an assignment using the add method:

>>> dom = set(['u1', 'u2', 'u3', 'u4'])
>>> g4 = Assignment(dom)
>>> g4.add('x', 'u1')
{'x': 'u1'}

With no arguments, purge() is equivalent to clear() on a dictionary:

>>> g4.purge()
>>> g4
  • domain (set) – the domain of discourse
  • assign (list) – a list of (varname, value) associations


__init__(domain[, assign])
add(var, val) Add a new variable-value pair to the assignment, and update self.variant.
clear(() -> None.  Remove all items from D.)
fromkeys(...) v defaults to None.
get((k[,d]) -> D[k] if k in D, ...)
has_key((k) -> True if D has a key k, else False)
items(() -> list of D’s (key, value) pairs, ...)
iteritems(() -> an iterator over the (key, ...)
iterkeys(() -> an iterator over the keys of D)
keys(() -> list of D’s keys)
pop((k[,d]) -> v, ...) If key is not found, d is returned if given, otherwise KeyError is raised
popitem(() -> (k, v), ...) 2-tuple; but raise KeyError if D is empty.
purge([var]) Remove one or all keys (i.e.
setdefault((k[,d]) -> D.get(k,d), ...)
update(([E, ...) If E present and has a .keys() method, does: for k in E: D[k] = E[k]
values(() -> list of D’s values)


unicode_repr() <==> repr(x)