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:

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

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  # deprecated
depset1 | depset2  # deprecated
depset1.union(depset2)  # deprecated

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

Integer division operator is //

Integer division operator is now // instead of /. This aligns with Python 3 and it highlights the fact it is a floor division.

x = 7 / 2  # deprecated

x = 7 // 2  # x is 3
  • Flag: --incompatible_disallow_slash_operator
  • Default: false

FileType is deprecated

The FileType function is going away. The main use-case was as an argument to the rule function. It's no longer needed, you can simply pass a list of strings to restrict the file types the rule accepts.

  • Flag: --incompatible_disallow_filetype
  • Default: false

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

New args API

The Args object returned by ctx.actions.args() has dedicated methods for appending the contents of a list or depset to the command line. Previously these use cases were lumped into its add() method, resulting in a more cluttered API.

With this flag, add() only works for scalar values, and its deprecated parameters are disabled. To add many arguments at once you must use add_all() or add_joined() instead.

  • Flag: --incompatible_disallow_old_style_args_add
  • Default: false

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: true