Network constructor fails if file is a StringIO stream

[mbastani]

The constructor of Network class allows the file argument to be either a string (path to the touchstone) or a file-like object (as the comment here states).

However, passing a file-like stream to the constructor raises an AttributeError, as the following MWE shows:

import io
import skrf as rf

with open('path/to/touchstone/file', 'r')  as fh:
    contents = fh.read()

stream = io.StringIO(contents)

net = rf.Network(stream)  # <--- raises AttributeError

The issue is that StringIO does not have a name attribute. And it seems that knowing the file name is necessary for constructing the network. (As the number of ports need to be determined from the file extension.)

[FranzForstmayr]

Assign a name property on the stream as a quick workaround

stream = io.StringIO(contents)
stream.name = 'simple-network.s2p'
net = rf.Network(stream) 

Not sure, if this is a bug or a feature :)

[jhillairet]

It is a kind of feature let's say ;)

The documentation of Touchstone reading is under update in #587 but not merged yet. It will be:

        Examples
        --------
        From filename
        >>> t = rf.Touchstone('network.s2p')
        
        File encoding can be specified to help parsing the special characters:
        
        >>> t = rf.Touchstone('network.s2p', encoding='ISO-8859-1')
        From file-object
        >>> file = open('network.s2p')
        >>> t = rf.Touchstone(file)
        
        From a io.StringIO object
        
        >>> link = 'https://raw.githubusercontent.com/scikit-rf/scikit-rf/master/examples/basic_touchstone_plotting/horn antenna.s1p'
        >>> r = requests.get(link)
        >>> stringio = io.StringIO(r.text)
        >>> stringio.name = 'horn.s1p'  # must be provided for the Touchstone parser
        >>> horn = rf.Touchstone(stringio)

[mbastani]

I'm not sure if requiring the user to add a property to the (standard) StringIO is a good solution.

In fact, by definition, a file-like object is only required to implement read and write methods in its API. So, either the constructor should be changed to use only the read method, or the argument of the constructor must be asked to implement a specific (more stringent) API that includes a name property.

I understand that the complication arises because, according to the Touchstone File Format Specification, in v1.0 files, the number of ports is (indirectly) embedded into the file extension rather than being recorded inside the file.

[jhillairet]

I don't really like the idea of adding a mandatory name to pass. I'd rather prefer to document how creating a Network from a StringIO in the Network Tutorial. which anyway should be revised now to reflect the latest changes simplifying creating Network from other parameters than S. Any comment on this?