Opened 4 years ago

Last modified 4 years ago

## #18346 closed enhancement

# Easier handling of vertex labels in graph backends — at Initial Version

Reported by: | ncohen | Owned by: | |
---|---|---|---|

Priority: | major | Milestone: | sage-6.7 |

Component: | graph theory | Keywords: | |

Cc: | borassi, dcoudert, darij, vdelecroix | Merged in: | |

Authors: | Nathann Cohen | Reviewers: | |

Report Upstream: | N/A | Work issues: | |

Branch: | Commit: | ||

Dependencies: | Stopgaps: |

### Description

This ticket addresses the following problem:

In the

`CGraph`

backends, three`cdef`

functions are used all the time to deal with vertex labels, i.e.`get_vertex`

,`vertex_label`

,`check_vertex`

. They are always called with the same arguments, which are all attributes of the backend that calls it. They should be method, so that the arguments are not needlessly repeated.

What this branch does:

- The
`CGraph`

backends (i.e.`SparseBackend`

,`DenseBackend`

and`StaticSparseGraph`

) and`GenericGraphBackend`

are turned into cdef classes.

- The three functions
`get_vertex`

,`vertex_label`

,`check_vertex`

become methods of`CGraphBackend`

- It adds a method
`CGraphBackend.c_graph()`

to get the two cdef attributes`_cg`

and`_cg_rev`

. Turns out that some code in Sage accesses directly`G._backend._cg`

.

This should have been sufficient, but there were (unexpected) consequences:

- There is no automatic pickling for cdef classes (as instrospection does not
work), and the doctests do not pass if that does not work. Consequently, I
implemented a unique
`__reduce__`

function in`GenericGraphBackend`

which handles the pickling for all backends (sparse/dense/static/networkx). It also removes some duplicated code as a result.

This makes the code a bit clearer (and it is needed). Also, moving this pickling
function above is good for the future, for there will be many modifications in
the future to the data structures in Sage: I plan to merge `CGraph`

and
`GenericBackend`

into only one class (but that's for later).

Nathann

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