Clarify dependency specifiers extra special cases in the grammar

[danyeaw]

In the dependency specifiers specification, it calls out extra as a special case to keep backwards compatibility. However, the grammar doesn't reflect this special case. This PR is a clarification to update the grammar so that it matches the text in the specification:

1. extra == "name" and extra != "name" are valid expression
2. Other marker operators are not valid (like >=)

I also used the following test script to test that the grammar was correct.

Test Script

from parsley import makeGrammar

grammar = r"""
    wsp           = ' ' | '\t'
    version_cmp   = wsp* <'<=' | '<' | '!=' | '===' | '==' | '>=' | '>' | '~='>
    marker_cop    = (wsp* 'in') | (wsp* 'not' wsp+ 'in')
    marker_op     = version_cmp | marker_cop
    python_str_c  = (wsp | letter | digit | '(' | ')' | '.' | '{' | '}' |
                     '-' | '_' | '*' | '#' | ':' | ';' | ',' | '/' | '?' |
                     '[' | ']' | '!' | '~' | '`' | '@' | '$' | '%' | '^' |
                     '&' | '=' | '+' | '|' | '<' | '>' )
    dquote        = '"'
    squote        = '\''
    python_str    = (squote <(python_str_c | dquote)*>:s squote |
                     dquote <(python_str_c | squote)*>:s dquote) -> s
    env_var       = ('python_version' | 'python_full_version' |
                     'os_name' | 'sys_platform' | 'platform_release' |
                     'platform_system' | 'platform_version' |
                     'platform_machine' | 'platform_python_implementation' |
                     'implementation_name' | 'implementation_version' |
                     'extras' | 'dependency_groups')
    extra_op      = '==' | '!='
    extra_expr    = wsp* 'extra' wsp* extra_op:o wsp* python_str:s -> (o, 'extra', s)
    marker_var    = wsp* (env_var | python_str)
    marker_expr   = extra_expr
                  | marker_var:l marker_op:o marker_var:r -> (o, l, r)
                  | wsp* '(' marker:m wsp* ')' -> m
    marker_and    = marker_expr:l wsp* 'and' marker_expr:r -> ('and', l, r)
                  | marker_expr:m -> m
    marker_or     = marker_and:l wsp* 'or' marker_and:r -> ('or', l, r)
                  | marker_and:m -> m
    marker        = marker_or
"""

compiled = makeGrammar(grammar, {})

valid = [
    # basic comparisons
    ("python_version > '3.10'", "env_var on left"),
    ('os_name == "posix"', "double-quoted string"),
    ("sys_platform != 'win32'", "not-equal"),
    ("python_version >= '3.8'", "gte"),
    # parenthesized grouping
    ("(os_name == 'posix')", "simple parens"),
    (
        "(os_name == 'a' or os_name == 'b') and sys_platform == 'linux'",
        "parens override precedence",
    ),
    # operator precedence: (a and b) or c
    ("os_name == 'a' and os_name == 'b' or os_name == 'c'", "and before or"),
    # operator precedence: a and (b or c)
    ("os_name == 'a' and (os_name == 'b' or os_name == 'c')", "explicit or in parens"),
    ("'SMP' in platform_version", "in comparison"),
    ('"dev" not in dependency_groups', "not in"),
    # extra field: only == and != are defined (set-like semantics)
    ("extra == 'toml'", "extra equality (set-like)"),
    ("extra != 'toml'", "extra inequality (set-like)"),
    ("'3.10' < python_version", "yoda 1"),
    ("'posix' == os_name", "yoda 2"),
    ("'win32' != sys_platform", "yoda 3"),
]

rejected = [
    # extra: ordered comparisons are not defined
    "extra > 'toml'",
    "extra >= 'toml'",
    "extra < 'toml'",
    "extra ~= 'toml'",
]

print("=== Valid expressions (should all parse) ===")
passed = 0
failed = 0
for expr, description in valid:
    try:
        result = compiled(expr).marker()
        print(f"  PASS  {description!r}: {expr!r} -> {result}")
        passed += 1
    except Exception as e:
        print(f"  FAIL  {description!r}: {expr!r} raised {e}")
        failed += 1

print()
print("=== These specifiers should all be rejected ===")
for expr in rejected:
    try:
        result = compiled(expr).marker()
        print(f"  FAIL  Yoda accepted: {expr!r} -> {result}")
        failed += 1
    except Exception:
        print(f"  PASS  Correctly rejected: {expr!r}")
        passed += 1

print()
print(f"Results: {passed} passed, {failed} failed")

---

📚 Documentation preview 📚: https://python-packaging-user-guide--2054.org.readthedocs.build/en/2054/