Enforce expression immutability in expr.args
#3099
Merged
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Fixes #3096, superceeds #3098
Summary/Motivation:
This resolves #3096 by closing a loophole in the "immutability" of expressions. For efficiency,
SumExpression
makes use of a mutable data structure (list) for storing the node arguments. Prior to this PR, we would return the raw underlying argument list fromSumExpression.args
. There are, however, edge cases where modifying either the expression or an entangled expression can cause that list to change. This PR changes the SumExpression API so that SumExpression.args always makes a copy of the_args_
list, thereby ensuring that the interface preserves the immutability that users expect.Note that this PR has a negative effect on performance in the form of a ~1% slow-down on LP and NL writers (as observed in the performance test suite).
Changes proposed in this PR:
SumExpression.args
now always returns a copy of the underlying argument listLegal Acknowledgement
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