site/docs/skylark/concepts.md

---
layout: documentation
title: Extensions - Overview
---
# Overview


## Loading an extension

Extensions are files with the `.bzl` extension. Use the `load` statement to
import a symbol from an extension.

```python
load("//build_tools/rules:maprule.bzl", "maprule")
```

This code will load the file `build_tools/rules/maprule.bzl` and add the
`maprule` symbol to the environment. This can be used to load new rules,
functions or constants (e.g. a string, a list, etc.). Multiple symbols can be
imported by using additional arguments to the call to `load`. Arguments must
be string literals (no variable) and `load` statements must appear at
top-level, i.e. they cannot be in a function body.

`load` also supports aliases, i.e. you can assign different names to the
imported symbols.

```python
load("//build_tools/rules:maprule.bzl", maprule_alias = "maprule")
```

You can define multiple aliases within one `load` statement. Moreover, the
argument list can contain both aliases and regular symbol names. The following
example is perfectly legal (please note when to use quotation marks).

```python
load(":my_rules.bzl", "some_rule", nice_alias = "some_other_rule")
```

In a `.bzl` file, symbols starting with `_` are private and cannot be loaded
from another file. Visibility doesn't affect loading (yet): you don't need to
use `exports_files` to make a `.bzl` file visible.

## Macros and rules

A [macro](macros.md) is a function that instantiates rules. It is useful when a
BUILD file is getting too repetitive or too complex, as it allows you to reuse
some code. The function is evaluated as soon as the BUILD file is read. After
the evaluation of the BUILD file, Bazel has little information about macros: if
your macro generates a `genrule`, Bazel will behave as if you wrote the
`genrule`. As a result, `bazel query` will only list the generated `genrule`.

A [rule](rules.md) is more powerful than a macro. It can access Bazel internals
and have full control over what is going on. It may for example pass information
to other rules.

If you want to reuse simple logic, start with a macro. If a macro becomes
complex, it is often a good idea to make it a rule. Support for a new language
is typically done with a rule. Rules are for advanced users: we expect that most
people will never have to write one, they will only load and call existing
rules.

## Evaluation model

A build consists of three phases.

* **Loading phase**. First, we load and evaluate all extensions and all BUILD
  files that are needed for the build. The execution of the BUILD files simply
  instantiates rules (each time a rule is called, it gets added to a graph).
  This is where macros are evaluated.

* **Analysis phase**. The code of the rules is executed (their `implementation`
  function), and actions are instantiated. An action describes how to generate
  a set of outputs from a set of inputs, e.g. "run gcc on hello.c and get
  hello.o". It is important to note that we have to list explicitly which
  files will be generated before executing the actual commands. In other words,
  the analysis phase takes the graph generated by the loading phase and
  generates an action graph.

* **Execution phase**. Actions are executed, when at least one of their outputs is
  required. If a file is missing or if a command fails to generate one output,
  the build fails. Tests are also run during this phase.

Bazel uses parallelism to read, parse and evaluate the `.bzl` files and `BUILD`
files. A file is read at most once per build and the result of the evaluation is
cached and reused. A file is evaluated only once all its dependencies (`load()`
statements) have been resolved. By design, loading a `.bzl` file has no visible
side-effect, it only defines values and functions.

Bazel tries to be clever: it uses dependency analysis to know which files must
be loaded, which rules must be analyzed, and which actions must be executed. For
example, if a rule generates actions that we don't need for the current build,
they will not be executed.

## Backward-incompatible 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:

*   build your code with the flag `--all_incompatible_changes`, or
*   use boolean flags to enable/disable specific incompatible changes.

This following are the planned incompatible changes that are implemented and
guarded behind flags.

### Set constructor

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](depsets.md) data structure and Python's set datatype.

*   Flag: `--incompatible_disallow_set_constructor`
*   Default: `false`

### Keyword-only arguments

Keyword-only parameters are parameters that can be called only using their name.

``` python
def foo(arg1, *, arg2): pass

foo(3, arg2=3)
```

``` python
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: `false`

### Mutating `+=`

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.

``` python
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.

*   Flag: `--incompatible_list_plus_equals_inplace`
*   Default: `false`

### 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 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.

``` python
load("/path/foo", "var")  # deprecated
load("//path:foo.bzl", "var")  # recommended
```

*   Flag: `--incompatible_load_argument_is_label`
*   Default: `false`

### 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: `false`

### 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.

``` python
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: `false`


## Upcoming changes

The following items are upcoming changes.

*   Comprehensions currently "leak" the values of their loop variables into the
    surrounding scope (Python 2 semantics). This will be changed so that
    comprehension variables are local (Python 3 semantics).

*   Previously dictionaries were guaranteed to use sorted order for their keys.
    Going forward, there is no guarantee on order besides that it is
    deterministic. As an implementation matter, some kinds of dictionaries may
    continue to use sorted order while others may use insertion order.

These changes concern the `load()` syntax in particular.

* Currently a `load()` statement can appear anywhere in a file so long as it is
  at the top-level (not in an indented block of code). In the future they will
  be required to appear at the beginning of the file, i.e., before any
  non-`load()` statement.

* In BUILD files, `load()` can overwrite an existing variable with the loaded
  symbol. This will be disallowed in order to improve consistency with .bzl
  files. Use load aliases to avoid name clashes.

* The .bzl file can be specified as either a path or a label. In the future only
  the label form will be allowed.

* Cross-package visibility restrictions do not yet apply to loaded .bzl files.
  At some point this will change. In order to load a .bzl from another package
  it will need to be exported, such as by using an `exports_files` declaration.
  The exact syntax has not yet been decided.


## Profiling the code

To profile your code and analyze the performance, use the `--profile` flag:

```
$ bazel build --nobuild --profile=/tmp/prof //path/to:target
$ bazel analyze-profile /tmp/prof --html --html_details
```

Then, open the generated HTML file (`/tmp/prof.html` in the example).