Coerce a single value to a list when serializing list variables

[uttam12331]

Summary

serialize_value assumes the value given for a list type is already a list and iterates it:

elif isinstance(type_, GraphQLList):
    return [serialize_value(inner_type, v) for v in value]

So a single value passed for a list variable is taken apart instead of being wrapped. A string is split into its characters and sent to the server without any error:

>>> serialize_variable_values(schema, document, {"ids": "abc"})
{'ids': ['a', 'b', 'c']}      # expected {'ids': ['abc']}
>>> serialize_variable_values(schema, document, {"ns": 5})
TypeError: 'int' object is not iterable    # expected {'ns': [5]}

The string case is the worrying one — no exception, just wrong data on the wire.

Fix

The GraphQL spec's input coercion says a non-list value provided for a list type is coerced to a list of one, and graphql-core implements exactly that in coerce_input_value:

if is_list_type(type_):
    item_type = type_.of_type
    if not is_iterable(input_value):
        # Lists accept a non-list value as a list of one.
        coerced_item = coerce_input_value(input_value, item_type)
        ...
        return [coerced_item]

This mirrors it using the same graphql.pyutils.is_iterable helper — the module this file already imports inspect from. is_iterable also excludes str and bytes, which is precisely the silently-wrong case above.

After the change:

{"ids": "abc"}      -> {'ids': ['abc']}
{"ns": 5}           -> {'ns': [5]}
{"ids": ["a", "b"]} -> {'ids': ['a', 'b']}   # unchanged
{"ids": None}       -> {'ids': None}         # unchanged

Tests

Added test_serialize_variables_single_value_for_list_type in tests/test_graphql_request.py, which fails before this change and passes after.

pytest tests/test_graphql_request.py tests/custom_scalars/ → 57 passed. The 2 failures in custom_scalars/test_money.py (sync_transport) also fail on a clean checkout here — they need transport deps I don't have locally, and are unrelated to this change. black, flake8 and isort are clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fb869a) to head (91e9d4b).
⚠️ Report is 75 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #611    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           38        40     +2     
  Lines         2908      3329   +421     
==========================================
+ Hits          2908      3329   +421     

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