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| -rw-r--r-- | site/docs/skylark/performance.md | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/site/docs/skylark/performance.md b/site/docs/skylark/performance.md index 3eb011f8bb..b7a88b4994 100644 --- a/site/docs/skylark/performance.md +++ b/site/docs/skylark/performance.md @@ -86,6 +86,28 @@ of overlapping targets. This happens when building your tests iterate a depset (explicitly or implicitly), or take its size, you are effectively calling `to_list`. This functionality will soon be removed. +### Never call `len(depset)` + +It is O(N) to get the number of items in a depset. It is however +O(1) to check if a depset is empty. This includes checking the truthiness +of a depset: + +``` +def _impl(ctx): + args = ctx.actions.args() + files = depset(...) + + # Bad, has to iterate over entire depset to get length + if len(files) == 0: + args.add("--files") + args.add(files) + + # Good, O(1) + if files: + args.add("--files") + args.add(files) +``` + ## Use `ctx.actions.args()` for command lines When building command lines you should use [ctx.actions.args()](lib/Args.html). |