Python interface: make the graph import functions accept URLs or Python file objects

[Tamás Nepusz]

Originally suggested by Chris Wj.

The igraph functions expect a FILE* and they want to call 'fread' (fscanf, etc.) on it. A possible way to go is to write the string to a temporary file and read that back -- another alternative is fmemopen() or open_memstream(), but we have to implement the temporary file solution anyway because of portability issues. [...] I have two possibilities in mind:

1. We could add another function to convert.c (say, igraphmodule_PyString_or_PyFile_to_fileptr()) that always returns an appropriate file pointer -- although we also have to take care of cleaning up the temporary file at some point.

2. We could simply override IO-related methods one level "above" in the Python layer (remember, the C layer implemeents igraph.core.GraphCore, igraph.Graph is inherited from that) to handle strings and files appropriately. This way we could maybe also handle URLs (i.e., if the string given is a URL, igraph could open a network connection and use the socket).

[Gábor Csárdi (gabor.csardi)]

The development of igraph has moved to github, so please do not comment on this bug here. You are of course welcome to comment on github, here:
https://github.com/igraph/igraph/issues/384