Backward compatibility

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:

  1. The backward incompatible change is introduced behind a flag with its default value set to false.
  2. In a later release, the flag's default value will be set to true. You can still use the flag to disable the change.
  3. Then in a later release, the flag will be removed and you will no longer be able to disable the change.

To check if your code will be compatible with future releases you can:

  • Build your code with the flag --all_incompatible_changes. This flag enables all backward incompatible changes, and so you can ensure your code is compatible with upcoming changes.
  • Use boolean flags to enable/disable specific backward incompatible changes.

Current backward incompatible changes

The following are the backward incompatible changes that are implemented and guarded behind flags in the current release:

Set constructor

To 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structure and Python's native set datatype (which does not currently exist in Skylark), the set constructor has been superseded by depset.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.

  • Flag: --incompatible_disallow_uncalled_set_constructor
  • Default: true

Keyword-only arguments

Keyword-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).

  • Flag: --incompatible_disallow_keyword_only_args
  • Default: true

Dictionary concatenation

We 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.

  • Flag: --incompatible_disallow_dict_plus
  • Default: false

Load must appear at top of file

Previously, 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.

  • Flag: --incompatible_bzl_disallow_load_after_statement
  • Default: false

Load argument is a label

Historically, 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
  • Flag: --incompatible_load_argument_is_label
  • Default: true

Top level if statements

This 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.

  • Flag: --incompatible_disallow_toplevel_if_statement
  • Default: true

Comprehensions variables

This 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.

  • Flag: --incompatible_comprehension_variables_do_not_leak
  • Default: true

Depset is no longer iterable

When 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
  • Flag: --incompatible_depset_is_not_iterable
  • Default: false

Depset union

To merge two sets, the following examples used to be supported, but are now deprecated:

depset1 + depset2
depset1 | depset2

The recommended solution is to use the depset constructor:

depset(transtive=[depset1, depset2])

See the depset documentation for more information.

  • Flag: --incompatible_depset_union
  • Default: false

String is no longer iterable

When 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"])
  name = "hello",
  srcs = "",

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
  • Flag: --incompatible_string_is_not_iterable
  • Default: false

Dictionary literal has no duplicates

When 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.

  • Flag: --incompatible_dict_literal_has_no_duplicates
  • Default: true

New actions API

This 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 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(...)
  • Flag: --incompatible_new_actions_api
  • Default: false

Checked arithmetic

When set, arithmetic operations (+, -, *) will fail in case of overflow. All integers are stored using signed 32 bits.

  • Flag: --incompatible_checked_arithmetic
  • Default: true

Glob tracking

When set, glob tracking is disabled. This is a legacy feature that we expect has no user-visible impact.

  • Flag: --incompatible_disable_glob_tracking
  • Default: false

Print statements

print 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.

  • Flag: --incompatible_show_all_print_messages
  • Default: true