Bazel is still in Beta and new releases may include backward incompatible changes. As we make changes and polish the extension mechanism, old features may be removed and new features that are not backward compatible may be added.
Backward incompatible changes are introduced gradually:
false.true. You can still use the flag to disable the change.To check if your code will be compatible with future releases you can:
--all_incompatible_changes. This flag enables all backward incompatible changes, and so you can ensure your code is compatible with upcoming changes.The following are the backward incompatible changes that are implemented and guarded behind flags in the current release:
if statementsTo maintain a clear distinction between the specialized depset data structure and Python's native set datatype (which does not currently exist in Skylark), the set constructor has been superseded by depset. It is no longer allowed to run code that calls the old set constructor.
However, for a limited time, it will not be an error to reference the set constructor from code that is not executed (e.g. a function that is never called). Enable this flag to confirm that your code does not still refer to the old set constructor from unexecuted code.
--incompatible_disallow_uncalled_set_constructortrueKeyword-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_argstrueWe 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_plusfalsePreviously, the load statement could appear anywhere in a .bzl file so long as it was at the top level. With this change, for .bzl files, load must appear at the beginning of the file, i.e. before any other non-load statement.
--incompatible_bzl_disallow_load_after_statementfalseHistorically, 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_labeltrueif 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_leaktrueWhen 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_iterablefalseTo merge two sets, the following examples used to be supported, but are now deprecated:
depset1 + depset2 depset1 | depset2 depset1.union(depset2)
The recommended solution is to use the depset constructor:
depset(transtive=[depset1, depset2])
See the depset documentation for more information.
--incompatible_depset_unionfalseWhen 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 # equivalent to: my_macro("hello", ["f", "o", "o", ".", "c", "c"]) my_macro( name = "hello", srcs = "foo.cc", )
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_duplicatestrueThis change removes the old methods for registering actions within rules, and requires that you use the new methods instead. The deprecated methods and their replacements are as follows.
ctx.new_file(...) --> ctx.actions.declare_file(...)ctx.experimental_new_directory(...) --> ctx.actions.declare_directory(...)ctx.action(...) --> either ctx.actions.run(...) or ctx.actions.run_shell(...)ctx.file_action(...) --> ctx.actions.write(...)ctx.empty_action(...) --> ctx.actions.do_nothing(...)ctx.template_action(...) --> ctx.actions.expand_template(...)--incompatible_new_actions_apifalseWhen set, arithmetic operations (+, -, *) will fail in case of overflow. All integers are stored using signed 32 bits.
--incompatible_checked_arithmetictrueWhen set, glob tracking is disabled. This is a legacy feature that we expect has no user-visible impact.
--incompatible_disable_glob_trackingfalseprint statements in Skylark code are supposed to be used for debugging only. Messages they yield used to be filtered out so that only messages from the same package as the top level target being built were shown by default (it was possible to override by providing, for example, --output_filter=). That made debugging hard. When the flag is set to true, all print messages are shown in the console without exceptions.
--incompatible_show_all_print_messagestrue