Lazy imports

This module allows one to lazily import objects into a namespace, where the actual import is delayed until the object is actually called or inspected. This is useful for modules that are expensive to import or may cause circular references, though there is some overhead in its use.

EXAMPLES:

sage: from sage.misc.lazy_import import lazy_import
sage: lazy_import('sage.rings.all', 'ZZ')
sage: type(ZZ)
<type 'sage.misc.lazy_import.LazyImport'>
sage: ZZ(4.0)
4

By default, a warning is issued if a lazy import module is resolved during Sage’s startup. In case a lazy import’s sole purpose is to break a circular reference and it is known to be resolved at startup time, one can use the at_startup option:

sage: lazy_import('sage.rings.all', 'ZZ', at_startup=True)

This option can also be used as an intermediate step toward not importing by default a module that is used in several places, some of which can already afford to lazy import the module but not all.

A lazy import that is marked as “at_startup” will print a message if it is actually resolved after the startup, so that the developer knows that (s)he can remove the flag:

sage: ZZ
Option ``at_startup=True`` for lazy import ZZ not needed anymore
Integer Ring

See also

lazy_import(), LazyImport

AUTHOR:

• Robert Bradshaw

class sage.misc.lazy_import.LazyImport

Bases: object

EXAMPLES:

sage: from sage.misc.lazy_import import LazyImport
sage: my_integer = LazyImport('sage.rings.all', 'Integer')
sage: my_integer(4)
4
sage: my_integer('101', base=2)
5
sage: my_integer(3/2)
Traceback (most recent call last):
...
TypeError: no conversion of this rational to integer

sage.misc.lazy_import.finish_startup()

This function must be called exactly once at the end of the Sage import process

sage.misc.lazy_import.get_star_imports(module_name)

Lookup the list of names in a module that would be imported with “import *” either via a cache or actually importing.

EXAMPLES:

sage: from sage.misc.lazy_import import get_star_imports
sage: 'get_star_imports' in get_star_imports('sage.misc.lazy_import')
True
sage: 'EllipticCurve' in get_star_imports('sage.schemes.all')
True

sage.misc.lazy_import.is_during_startup()

Return whether Sage is currently starting up.

OUTPUT:

Boolean

sage.misc.lazy_import.lazy_import(module, names, as_=None, at_startup=False, namespace=None, deprecation=None, feature=None)

Create a lazy import object and inject it into the caller’s global namespace. For the purposes of introspection and calling, this is like performing a lazy “from module import name” where the import is delayed until the object actually is used or inspected.

INPUT:

• module – a string representing the module to import

• names – a string or list of strings representing the names to import from module

• as_ – (optional) a string or list of strings representing the names of the objects in the importing module. This is analogous to from ... import ... as ....

• at_startup – a boolean (default: False); whether the lazy import is supposed to be resolved at startup time

• namespace – the namespace where importing the names; by default, import the names to current namespace

• deprecation – (optional) if not None, a deprecation warning will be issued when the object is actually imported; deprecation should be either a trac number (integer) or a pair (trac_number, message)

• feature – a python module (optional), if it cannot be imported an appropriate error is raised

See also

sage.misc.lazy_import, LazyImport

EXAMPLES:

sage: from sage.misc.lazy_import import lazy_import
sage: lazy_import('sage.rings.all', 'ZZ')
sage: type(ZZ)
<type 'sage.misc.lazy_import.LazyImport'>
sage: ZZ(4.0)
4
sage: lazy_import('sage.rings.all', 'RDF', 'my_RDF')
sage: my_RDF._get_object() is RDF
True
sage: my_RDF(1/2)
0.5

sage: lazy_import('sage.all', ['QQ', 'RR'], ['my_QQ', 'my_RR'])
sage: my_QQ._get_object() is QQ
True
sage: my_RR._get_object() is RR
True

Upon the first use, the object is injected directly into the calling namespace:

sage: lazy_import('sage.all', 'ZZ', 'my_ZZ')
sage: my_ZZ is ZZ
False
sage: my_ZZ(37)
37
sage: my_ZZ is ZZ
True

We check that lazy_import() also works for methods:

sage: class Foo(object):
....:     lazy_import('sage.all', 'plot')
sage: class Bar(Foo):
....:     pass
sage: type(Foo.__dict__['plot'])
<type 'sage.misc.lazy_import.LazyImport'>
sage: 'EXAMPLES' in Bar.plot.__doc__
True
sage: type(Foo.__dict__['plot'])
<... 'function'>

If deprecated then a deprecation warning is issued:

sage: lazy_import('sage.all', 'Qp', 'my_Qp', deprecation=14275)
sage: my_Qp(5)
doctest:...: DeprecationWarning:
Importing my_Qp from here is deprecated. If you need to use it, please import it directly from sage.all
See http://trac.sagemath.org/14275 for details.
5-adic Field with capped relative precision 20

An example of deprecation with a message:

sage: lazy_import('sage.all', 'Qp', 'my_Qp_msg', deprecation=(14275, "This is an example."))
sage: my_Qp_msg(5)
doctest:...: DeprecationWarning: This is an example.
See http://trac.sagemath.org/14275 for details.
5-adic Field with capped relative precision 20

An example of an import relying on a feature:

sage: from sage.features import PythonModule
sage: lazy_import('ppl', 'equation', feature=PythonModule('ppl', spkg='pplpy'))
sage: equation
<built-in function equation>
sage: lazy_import('PyNormaliz', 'NmzListConeProperties', feature=PythonModule('PyNormaliz', spkg='pynormaliz'))  # optional - pynormaliz
sage: NmzListConeProperties  # optional - pynormaliz
<built-in function NmzListConeProperties>
sage: lazy_import('foo', 'not_there', feature=PythonModule('foo', spkg='non-existing-package'))
sage: not_there
Failed lazy import:
foo is not available.
Importing not_there failed: No module named 'foo'...
No equivalent system packages for ... are known to Sage...

sage.misc.lazy_import.save_cache_file()

Used to save the cached * import names.

sage.misc.lazy_import.test_fake_startup()

For testing purposes only.

Switch the startup lazy import guard back on.

EXAMPLES:

sage: sage.misc.lazy_import.test_fake_startup()
sage: from sage.misc.lazy_import import lazy_import
sage: lazy_import('sage.rings.all', 'ZZ', 'my_ZZ')
sage: my_ZZ(123)
Resolving lazy import ZZ during startup
123
sage: sage.misc.lazy_import.finish_startup()