Variants of getattr()

class sage.cpython.getattr.AttributeErrorMessage

Bases: object

Tries to emulate the standard Python AttributeError message.

Note

The typical fate of an attribute error is being caught. Hence, under normal circumstances, nobody will ever see the error message. The idea for this class is to provide an object that is fast to create and whose string representation is an attribute error’s message. That string representation is only created if someone wants to see it.

EXAMPLES:

sage: 1.bla  #indirect doctest
Traceback (most recent call last):
...
AttributeError: 'sage.rings.integer.Integer' object has no attribute 'bla'
sage: QQ[x].gen().bla
Traceback (most recent call last):
...
AttributeError: 'sage.rings.polynomial.polynomial_rational_flint.Polynomial_rational_flint' object has no attribute 'bla'
sage: from sage.cpython.getattr import AttributeErrorMessage
sage: AttributeErrorMessage(int(1), 'bla')
'int' object has no attribute 'bla'

AUTHOR:

  • Simon King (2011-05-21)
cls
name
sage.cpython.getattr.dir_with_other_class(self, cls)

Emulates dir(self), as if self was also an instance cls, right after caller_class in the method resolution order (self.__class__.mro())

EXAMPLES:

sage: class A(object):
....:    a = 1
....:    b = 2
....:    c = 3
sage: class B(object):
....:    b = 2
....:    c = 3
....:    d = 4
sage: x = A()
sage: x.c = 1; x.e = 1
sage: from sage.cpython.getattr import dir_with_other_class
sage: dir_with_other_class(x, B)
[..., 'a', 'b', 'c', 'd', 'e']

Check that objects without dicts are well handled:

sage: cython("cdef class A:\n    cdef public int a")
sage: cython("cdef class B:\n    cdef public int b")
sage: x = A()
sage: x.a = 1
sage: hasattr(x,'__dict__')
False
sage: dir_with_other_class(x, B)
[..., 'a', 'b']
sage.cpython.getattr.getattr_from_other_class(self, cls, name)

Emulate getattr(self, name), as if self was an instance of cls.

INPUT:

  • self – some object
  • cls – a new-style class
  • name – a string

If self is an instance of cls, raises an AttributeError, to avoid a double lookup. This function is intended to be called from __getattr__, and so should not be called if name is an attribute of self.

EXAMPLES:

sage: from sage.cpython.getattr import getattr_from_other_class
sage: class A(object):
....:      def inc(self):
....:          return self + 1
....:
....:      @staticmethod
....:      def greeting():
....:          print("Hello World!")
....:
....:      @lazy_attribute
....:      def lazy_attribute(self):
....:          return repr(self)
sage: getattr_from_other_class(1, A, "inc")
<bound method A.inc of 1>
sage: getattr_from_other_class(1, A, "inc")()
2

Static methods work:

sage: getattr_from_other_class(1, A, "greeting")()
Hello World!

Caveat: lazy attributes work with extension types only if they allow attribute assignment or have a public attribute __cached_methods of type <dict>. This condition is satisfied, e.g., by any class that is derived from Parent:

sage: getattr_from_other_class(1, A, "lazy_attribute")
Traceback (most recent call last):
...
AttributeError: 'sage.rings.integer.Integer' object has no attribute 'lazy_attribute'

The integer ring is a parent, so, lazy attributes work:

sage: getattr_from_other_class(ZZ, A, "lazy_attribute")
'Integer Ring'
sage: getattr_from_other_class(PolynomialRing(QQ, name='x', sparse=True).one(), A, "lazy_attribute")
'1'
sage: getattr_from_other_class(17, A, "lazy_attribute")
Traceback (most recent call last):
...
AttributeError: 'sage.rings.integer.Integer' object has no attribute 'lazy_attribute'

In general, descriptors are not yet well supported, because they often do not accept to be cheated with the type of their instance:

sage: A.__weakref__.__get__(1)
Traceback (most recent call last):
...
TypeError: descriptor '__weakref__' for 'A' objects doesn't apply
to 'sage.rings.integer.Integer' object

When this occurs, an AttributeError is raised:

sage: getattr_from_other_class(1, A, "__weakref__")
Traceback (most recent call last):
...
AttributeError: 'sage.rings.integer.Integer' object has no attribute '__weakref__'

This was caught by trac ticket #8296 for which we do a couple more tests:

sage: "__weakref__" in dir(A)
True
sage: "__weakref__" in dir(1)  # py2
False
sage: 1.__weakref__
Traceback (most recent call last):
...
AttributeError: 'sage.rings.integer.Integer' object has no attribute '__weakref__'

sage: n = 1
sage: ip = get_ipython()                 # not tested: only works in interactive shell
sage: ip.magic_psearch('n.N')            # not tested: only works in interactive shell
n.N
sage: ip.magic_psearch('n.__weakref__')  # not tested: only works in interactive shell

Caveat: When __call__ is not defined for instances, using A.__call__ yields the method __call__ of the class. We use a workaround but there is no guarantee for robustness.

sage: getattr_from_other_class(1, A, “__call__”) Traceback (most recent call last): … AttributeError: ‘sage.rings.integer.Integer’ object has no attribute ‘__call__’
sage.cpython.getattr.raw_getattr(obj, name)

Like getattr(obj, name) but without invoking the binding behavior of descriptors under normal attribute access. This can be used to easily get unbound methods or other descriptors.

This ignores __getattribute__ hooks but it does support __getattr__.

Note

For Cython classes, __getattr__ is actually implemented as __getattribute__, which means that it is not supported by raw_getattr.

EXAMPLES:

sage: class X:
....:     @property
....:     def prop(self):
....:         return 42
....:     def method(self):
....:         pass
....:     def __getattr__(self, name):
....:         return "magic " + name
sage: raw_getattr(X, "prop")
<property object at ...>
sage: raw_getattr(X, "method")
<function ...method at ...>
sage: raw_getattr(X, "attr")
Traceback (most recent call last):
...
AttributeError: '...' object has no attribute 'attr'
sage: x = X()
sage: raw_getattr(x, "prop")
<property object at ...>
sage: raw_getattr(x, "method")
<function ...method at ...>
sage: raw_getattr(x, "attr")
'magic attr'
sage: x.__dict__["prop"] = 'no'
sage: x.__dict__["method"] = 'yes'
sage: x.__dict__["attr"] = 'ok'
sage: raw_getattr(x, "prop")
<property object at ...>
sage: raw_getattr(x, "method")
'yes'
sage: raw_getattr(x, "attr")
'ok'

The same tests with an inherited new-style class:

sage: class Y(X, object):
....:     pass
sage: raw_getattr(Y, "prop")
<property object at ...>
sage: raw_getattr(Y, "method")
<function ...method at ...>
sage: raw_getattr(Y, "attr")
Traceback (most recent call last):
...
AttributeError: '...' object has no attribute 'attr'
sage: y = Y()
sage: raw_getattr(y, "prop")
<property object at ...>
sage: raw_getattr(y, "method")
<function ...method at ...>
sage: raw_getattr(y, "attr")
'magic attr'
sage: y.__dict__["prop"] = 'no'
sage: y.__dict__["method"] = 'yes'
sage: y.__dict__["attr"] = 'ok'
sage: raw_getattr(y, "prop")
<property object at ...>
sage: raw_getattr(y, "method")
'yes'
sage: raw_getattr(y, "attr")
'ok'