Added support for 'file://' URL schema on the RefResolver

[USSX-Hares]

Fixes #398

Difference

Before this fix, RefResolver created with base_uri starting with file:// could work not properly.

Example code to test:

import os.path
import json

from jsonschema import validate, RefResolver

contact = \
{
    "name": "William Johns",
    "age": 25,
    "birthDate": { "month": "apr", "day": 15 }
}

def main():
    schema_path = "schemas/main_schema.json"
    schema = json.load(open(schema_path))
    validate(contact, schema, resolver=RefResolver(base_uri='file://' + os.path.dirname(os.path.abspath(schema_path)), referrer=schema))

if (__name__ == '__main__'):
    main()

Work tree:

    /
    +-- code.py
    +-- schemas /
    |   +-- dependencies /
    |   |   +-- date.json
    |   |   +-- month.json
    |   |   +-- positive_integer.json
    |   +-- main_schema.json

$ref Dependencies:

main_schema => date, positive_integer
date => month, positive_integer

Tests

Tests passed on:

• Windows 7, Python 2.7.15
• Windows 7, Python 3.5.2
• Windows 7, Python 3.7.2

Would also test on:

• Linux (Ubuntu 16.04), Python 2.7.
• Linux (Ubuntu 16.04), Python 3.5.2
• Linux (Ubuntu 16.04), Python 3.6.7
• Linux (Ubuntu 16.04), Python 3.7.2

[codecov]

Codecov Report

Merging #531 into master will decrease coverage by 0.07%.
The diff coverage is 87.5%.

@@            Coverage Diff            @@
##           master    #531      +/-   ##
=========================================
- Coverage   96.27%   96.2%   -0.08%     
=========================================
  Files          19      19              
  Lines        2416    2422       +6     
  Branches      314     316       +2     
=========================================
+ Hits         2326    2330       +4     
- Misses         77      78       +1     
- Partials       13      14       +1

[codecov]

Codecov Report

Merging #531 into master will decrease coverage by 0.07%.
The diff coverage is 87.5%.

@@            Coverage Diff            @@
##           master    #531      +/-   ##
=========================================
- Coverage   96.27%   96.2%   -0.08%     
=========================================
  Files          19      19              
  Lines        2416    2422       +6     
  Branches      314     316       +2     
=========================================
+ Hits         2326    2330       +4     
- Misses         77      78       +1     
- Partials       13      14       +1

[Julian]

Hi! Thanks.

I don't remember the behavior here off hand, but does this work with requests?

If so now that requests is as prevalent I'm more inclined to just drop support for ref resolution without it.

[USSX-Hares]

@Julian did not try to use requests for accessing local files. I'm not sure if this would work.