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.


sage: lazy_import('sage.rings.integer_ring', 'ZZ')
sage: type(ZZ)
<class 'sage.misc.lazy_import.LazyImport'>
sage: ZZ(4.0)

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.integer_ring', '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
UserWarning: Option ``at_startup=True`` for lazy import ZZ not needed anymore
Integer Ring


  • Robert Bradshaw

class sage.misc.lazy_import.LazyImport#

Bases: object


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

Return the private attributes of a LazyImport object in a dictionary.

This is for debugging and doctesting purposes only.


sage: from sage.misc.lazy_import import attributes
sage: lazy_import("sage.structure.unique_representation", "foo")
sage: attributes(foo)['_namespace'] is globals()
sage: D = attributes(foo)
sage: del D['_namespace']
sage: D
{'_as_name': 'foo',
 '_at_startup': False,
 '_deprecation': None,
 '_module': 'sage.structure.unique_representation',
 '_name': 'foo',
 '_object': None}

Adjust LazyImport bindings in given namespace to refer to this actual namespace.

When LazyImport objects are imported into other namespaces via normal import instructions, the data stored on a LazyImport object that helps it to adjust the binding in the namespace to the actual imported object upon access is not adjusted. This routine fixes that.


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


sage: # needs sage.symbolic
sage: from sage.misc.lazy_import import attributes, clean_namespace
sage: from sage.calculus.calculus import maxima as C
sage: attributes(C)['_as_name']
sage: attributes(C)['_namespace'] is sage.calculus.calculus.__dict__
sage: clean_namespace(globals())
sage: attributes(C)['_as_name']
sage: attributes(C)['_namespace'] is globals()

Make sure that the startup phase is finished.

In contrast to finish_startup(), this function can be called repeatedly.


Finish the startup phase.

This function must be called exactly once at the end of the Sage import process (all).


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


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

Return whether Sage is currently starting up.



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.


  • 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 (issue_number, message)

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


sage: lazy_import('sage.rings.integer_ring', 'ZZ')
sage: type(ZZ)
<class 'sage.misc.lazy_import.LazyImport'>
sage: ZZ(4.0)
sage: lazy_import('sage.rings.real_double', 'RDF', 'my_RDF')
sage: my_RDF._get_object() is RDF
sage: my_RDF(1/2)

sage: lazy_import('sage.rings.rational_field', ['QQ', 'frac'], ['my_QQ', 'my_frac'])
sage: my_QQ._get_object() is QQ
sage: my_frac._get_object() is sage.rings.rational_field.frac

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

sage: lazy_import('sage.rings.integer_ring', 'ZZ', 'my_ZZ')
sage: my_ZZ is ZZ
sage: my_ZZ(37)
sage: my_ZZ is ZZ

We check that lazy_import() also works for methods:

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

If deprecated then a deprecation warning is issued:

sage: lazy_import('sage.rings.padics.factory', 'Qp', 'my_Qp',
....:             deprecation=14275)
sage: my_Qp(5)                                                                  # needs sage.rings.padics
doctest:...: DeprecationWarning:
Importing my_Qp from here is deprecated; please use "from sage.rings.padics.factory import Qp as my_Qp" instead.
See for details.
5-adic Field with capped relative precision 20

An example of deprecation with a message:

sage: lazy_import('sage.rings.padics.factory', 'Qp', 'my_Qp_msg',
....:             deprecation=(14275, "This is an example."))
sage: my_Qp_msg(5)                                                              # needs sage.rings.padics
doctest:...: DeprecationWarning: This is an example.
See 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', type='standard'))
sage: equation                                                                  # needs pplpy
<cyfunction equation at ...>
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...

Used to save the cached * import names.


For testing purposes only.

Switch the startup lazy import guard back on.


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