Bazel is still in Beta and we are going to do breaking changes. As we make changes and polish the extension mechanism, old features may be removed and new features that are not backwards-compatible may be added.
Each release, new incompatible changes will be behind a flag with its default value set to false. In later releases, the flag will be enabled by default, or the flag will be removed entirely.
To check if your code will be compatible with future releases:
--all_incompatible_changes, orThis following are the planned incompatible changes that are implemented and guarded behind flags.
We are removing the set constructor. Use depset instead. set and depset are equivalent, you just need to do search and replace to update the old code.
We are doing this to reduce confusion between the specialized depset data structure and Python's set datatype.
--incompatible_disallow_set_constructorfalseKeyword-only parameters are parameters that can be called only using their name.
def foo(arg1, *, arg2): pass foo(3, arg2=3)
def bar(arg1, *rest, arg2): pass bar(3, arg2=3)
In both examples, arg2 must be named at the call site. To preserve syntactic compatibility with Python 2, we are removing this feature (which we have never documented).
--incompatible_disallow_keyword_only_argsfalse+=We are changing left += right when left is a list. The old behavior is equivalent to left = left + right, which creates a new list and assigns it to left. The new behavior does not rebind left, but instead just mutates the list in-place.
def fct(): li = [1] alias = li li += [2] # Old behavior: alias == [1] # New behavior: alias == [1, 2]
This change makes Skylark more compatible with Python and avoids performance issues. The += operator for tuples is unaffected.
--incompatible_list_plus_equals_inplacefalseWe are removing the + operator on dictionaries. This includes the += form where the left-hand side is a dictionary. This is done to improve compatibility with Python. A possible workaround is to use the .update method instead.
--incompatible_disallow_dict_plusfalseHistorically, the first argument of load could be a path with an implicit .bzl suffix. We are going to require that all load statements use the label syntax.
load("/path/foo", "var") # deprecated load("//path:foo.bzl", "var") # recommended
--incompatible_load_argument_is_labelfalseif statementsThis change forbids if statements at the top level of .bzl files (they are already forbidden in BUILD files). This change ensures that every global value has a single declaration. This restriction is consistent with the idea that global values cannot be redefined.
--incompatible_disallow_toplevel_if_statementtrueThis change makes list and dict comprehensions follow Python 3‘s semantics instead of Python 2’s. That is, comprehensions have their own local scopes, and variables bound by comprehensions are not accessible in the outer scope.
As a temporary measure to help detect breakage, this change also causes variables defined in the immediate outer scope to become inaccessible if they are shadowed by any variables in a comprehension. This disallows any uses of the variable's name where its meaning would differ under the Python 2 and Python 3 semantics. Variables above the immediate outer scope are not affected.
def fct(): x = 10 y = [x for x in range(3)] return x
The meaning of this program depends on the flag:
Under Skylark without this flag: x is 10 before the comprehension and 2 afterwards. (2 is the last value assigned to x while evaluating the comprehension.)
Under Skylark with this flag: x becomes inaccessible after the comprehension, so that return x is an error. If we moved the x = 10 to above the function, so that x became a global variable, then no error would be raised, and the returned number would be 10.
In other words, please do not refer to a loop variable outside the list or dict comprehension.
--incompatible_comprehension_variables_do_not_leakfalseWhen the flag is set to true, depset objects are not treated as iterable. If you need an iterable, call the .to_list() method. This affects for loops and many functions, e.g. list, tuple, min, max, sorted, all, and any. The goal of this change is to avoid accidental iteration on depset, which can be expensive.
deps = depset() [x.path for x in deps] # deprecated [x.path for x in deps.to_list()] # recommended sorted(deps) # deprecated sorted(deps.to_list()) # recommended
--incompatible_depset_is_not_iterablefalseWhen the flag is set to true, string objects are not treated as iterable. This affects for loops and many functions, e.g. list, tuple, min, max, sorted, all, and any. String iteration has been a source of errors and confusion, such as this error:
def my_macro(name, srcs): for src in srcs: # do something with src my_macro("foo") # equivalent to: my_macro(["f", "o", "o"])
String indexing and len are still allowed. If you need to iterate over a string, you may explicitly use:
my_string="hello world" for i in range(len(my_string)): char = my_string[i] # do something with char
--incompatible_string_is_not_iterablefalseWhen the flag is set to true, duplicated keys are not allowed in the dictionary literal syntax.
{"a": 2, "b": 3, "a": 4} # error
When the flag is false, the last value overrides the previous value (so the example above is equivalent to {"a": 4, "b": 3}. This behavior has been a source of bugs, which is why we are going to forbid it.
If you really want to override a value, use a separate statement: mydict["a"] = 4.
--incompatible_dict_literal_has_no_duplicatesfalseWhen set, arithmetic operations (+, -, *) will fail in case of overflow. All integers are stored using signed 32 bits.
--incompatible_incompatible_checked_arithmeticfalse