[MRG+1] Doc: binary mode is required for exporters

[elacuesta]

Closes #2553

I left XmlItemExporter out of the modifications, it's the only class which does not seem to have problems writing to either bytes or str streams (io.BytesIO or io.StringIO, for instance).

from io import BytesIO, StringIO

with BytesIO(b'') as f:
    exporter = XmlItemExporter(f)
    exporter.start_exporting()
    exporter.export_item({'some': 'value'})
    exporter.finish_exporting()
    print(f.getvalue())

with StringIO(u'') as f:
    exporter = XmlItemExporter(f)
    exporter.start_exporting()
    exporter.export_item({'some': 'value'})
    exporter.finish_exporting()
    print(f.getvalue())

outputs:

b'<?xml version="1.0" encoding="utf-8"?>\n<items><item><some>value</some></item></items>'
<?xml version="1.0" encoding="utf-8"?>
<items><item><some>value</some></item></items>

/cc @kmike

[kmike]

While XmlItemExporter works with text files, I'm not sure it is intentional. For example, 'encoding' argument doesn't work properly with XmlItemExporter + text files.

[kmike]

its

[elacuesta]

oops

[elacuesta]

Right, the following:

from io import StringIO

with StringIO(u'') as f:
    exporter = XmlItemExporter(f, encoding='utf8')
    exporter.start_exporting()
    exporter.export_item({'some': u'valûe'})
    exporter.finish_exporting()
    print(f.getvalue())

Fails in python 2, it seems like the binary mode is more consistent. I'll add the note to the XmlItemExporter as well. Thanks!

[codecov-io]

Codecov Report

Merging #2564 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2564   +/-   ##
=======================================
  Coverage   83.59%   83.59%           
=======================================
  Files         161      161           
  Lines        8812     8812           
  Branches     1296     1296           
=======================================
  Hits         7366     7366           
  Misses       1200     1200           
  Partials      246      246

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update afac3fd...922d3fe. Read the comment docs.

[kmike]

Looks good, thanks @elacuesta!

[elacuesta]

Thanks to you and @TDominiqueWilliams 😄