Embedding · Errors

Every exception lessish raises inherits from LessError, so one except clause catches anything the compiler can throw.

from lessish import Lessish, LessError

ls = Lessish()
try:
    ls.compile('.a { color: @undefined-name; }')
    ok = True
except LessError:
    ok = False
assert ok is False

The hierarchy

Exception	Raised when
`ParseError`	The lexer / parser refused the input.
`EvalError`	Base for evaluation-time failures. Subclasses below.
`UndefinedNameError`	A variable or mixin name doesn't resolve.
`OperationError`	Bad arithmetic (e.g. incoherent units under `strict_units`).
`TypeMismatchError`	A function/operator got the wrong value type.
`ArgumentError`	A mixin/function call's arguments don't match.
`UnsupportedFeatureError`	An out-of-scope construct (JS backticks, `@plugin`, remote `@import`) reached eval.
`FileError`	An `@import` target wasn't found.
`SecurityError`	The `file_io` policy refused a read.

UndefinedNameError, OperationError, TypeMismatchError, and ArgumentError are subclasses of EvalError; EvalError, UnsupportedFeatureError, FileError, and SecurityError are all subclasses of LessError.

from lessish import Lessish, LessError, EvalError, UndefinedNameError

ls = Lessish()
try:
    ls.compile('.a { color: @nope; }')
except UndefinedNameError as err:
    assert isinstance(err, EvalError)
    assert isinstance(err, LessError)

Location and snippet

Every LessError carries a .location — a SourceLocation with filename, line, column, and index — and, where the stage can produce one, a .snippet string with a caret pointing at the offending column.

from lessish import Lessish, ParseError

ls = Lessish()
try:
    ls.compile('.a { color: ;', filename='styles.less')
except ParseError as err:
    assert err.location.filename == 'styles.less'
    assert err.location.line == 1
    assert err.location.column == 14
    assert str(err.location) == 'styles.less:1:14'
    print(err.snippet)

The print(err.snippet) above produces:

   1 | .a { color: ;
                    ^

.snippet is best-effort: parse errors carry a caret snippet; some eval-time errors expose only .location (then .snippet is None). Always guard on .location, treat .snippet as optional.

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