# Nonstandard Import Conventions

This document covers ty-specific extensions to the
[standard import conventions](https://typing.python.org/en/latest/spec/distributing.html#import-conventions),
and other intentional deviations from actual python semantics.

This file currently covers the following details:

- **froms are locals**: a `from..import` can only define locals, it does not have global
    side-effects. Specifically any submodule attribute `a` that's implicitly introduced by either
    `from .a import b` or `from . import a as b` (in an `__init__.py(i)`) is a local and not a
    global. However we only introduce this symbol if the `from..import` is in global-scope. This
    means imports at the start of a file work as you'd expect, while imports in a function don't
    introduce submodule attributes.

- **first from first serve**: only the *first* `from..import` in an `__init__.py(i)` that imports a
    particular direct submodule of the current package introduces that submodule as a local.
    Subsequent imports of the submodule will not introduce that local. This reflects the fact that
    in actual python only the first import of a submodule (in the entire execution of the program)
    introduces it as an attribute of the package. By "first" we mean "the first time in global
    scope".

- **dot re-exports**: `from . import a` in an `__init__.pyi` is considered a re-export of `a`
    (equivalent to `from . import a as a`). This is required to properly handle many stubs in the
    wild. Equivalent imports like `from whatever.thispackage import a` also introduce a re-export
    (this has essentially zero ecosystem impact, we just felt it was more consistent). The only way
    to opt out of this is to rename the import to something else (`from . import a as b`).
    `from .a import b` and equivalent does *not* introduce a re-export.

Note: almost all tests in here have a stub and non-stub version, because we're interested in both
defining symbols *at all* and re-exporting them.

## Relative `from` Import of Direct Submodule in `__init__`

We consider the `from . import submodule` idiom in an `__init__.pyi` an explicit re-export.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from . import imported
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`mypackage/fails.pyi`:

```pyi
Y: int = 47
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
# error: [possibly-missing-submodule] "Submodule `fails` might not have been imported"
reveal_type(mypackage.fails.Y)  # revealed: Unknown
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from . import imported
```

`mypackage/imported.py`:

```py
X: int = 42
```

`mypackage/fails.py`:

```py
Y: int = 47
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
# error: [possibly-missing-submodule] "Submodule `fails` might not have been imported"
reveal_type(mypackage.fails.Y)  # revealed: Unknown
```

## Absolute `from` Import of Direct Submodule in `__init__`

If an absolute `from...import` happens to import a submodule (i.e. it's equivalent to
`from . import y`) we also treat it as a re-export.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from mypackage import imported
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`mypackage/fails.pyi`:

```pyi
Y: int = 47
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
# error: [possibly-missing-submodule] "Submodule `fails` might not have been imported"
reveal_type(mypackage.fails.Y)  # revealed: Unknown
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from mypackage import imported
```

`mypackage/imported.py`:

```py
X: int = 42
```

`mypackage/fails.py`:

```py
Y: int = 47
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
# error: [possibly-missing-submodule] "Submodule `fails` might not have been imported"
reveal_type(mypackage.fails.Y)  # revealed: Unknown
```

## Import of Direct Submodule in `__init__`

An `import` that happens to import a submodule does not expose the submodule as an attribute. (This
is an arbitrary decision and can be changed!)

### In Stub

`mypackage/__init__.pyi`:

```pyi
import mypackage.imported
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage

# TODO: this could work and would be nice to have?
# error: [possibly-missing-submodule] "Submodule `imported` might not have been imported"
reveal_type(mypackage.imported.X)  # revealed: Unknown
```

### In Non-Stub

`mypackage/__init__.py`:

```py
import mypackage.imported
```

`mypackage/imported.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage

# TODO: this could work and would be nice to have
# error: [possibly-missing-submodule] "Submodule `imported` might not have been imported"
reveal_type(mypackage.imported.X)  # revealed: Unknown
```

## Relative `from` Import of Nested Submodule in `__init__`

`from .submodule import nested` in an `__init__.pyi` does re-export `mypackage.submodule`, but not
`mypackage.submodule.nested` or `nested`.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from .submodule import nested
```

`mypackage/submodule/__init__.pyi`:

```pyi
```

`mypackage/submodule/nested.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.submodule)  # revealed: <module 'mypackage.submodule'>
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested.X)  # revealed: Unknown
# error: [unresolved-attribute] "has no member `nested`"
reveal_type(mypackage.nested)  # revealed: Unknown
# error: [unresolved-attribute] "has no member `nested`"
reveal_type(mypackage.nested.X)  # revealed: Unknown
```

### In Non-Stub

`from .submodule import nested` in an `__init__.py` exposes `mypackage.submodule` and `nested`.

`mypackage/__init__.py`:

```py
from .submodule import nested
```

`mypackage/submodule/__init__.py`:

```py
```

`mypackage/submodule/nested.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.submodule)  # revealed: <module 'mypackage.submodule'>
# TODO: this would be nice to support
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested.X)  # revealed: Unknown
reveal_type(mypackage.nested)  # revealed: <module 'mypackage.submodule.nested'>
reveal_type(mypackage.nested.X)  # revealed: int
```

## Absolute `from` Import of Nested Submodule in `__init__`

`from mypackage.submodule import nested` in an `__init__.pyi` does not re-export
`mypackage.submodule`, `mypackage.submodule.nested`, or `nested`.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from mypackage.submodule import nested
```

`mypackage/submodule/__init__.pyi`:

```pyi
```

`mypackage/submodule/nested.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.submodule)  # revealed: <module 'mypackage.submodule'>
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested.X)  # revealed: Unknown
# error: [unresolved-attribute] "has no member `nested`"
reveal_type(mypackage.nested)  # revealed: Unknown
# error: [unresolved-attribute] "has no member `nested`"
reveal_type(mypackage.nested.X)  # revealed: Unknown
```

### In Non-Stub

`from mypackage.submodule import nested` in an `__init__.py` creates both `submodule` and `nested`.

`mypackage/__init__.py`:

```py
from mypackage.submodule import nested
```

`mypackage/submodule/__init__.py`:

```py
```

`mypackage/submodule/nested.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.submodule)  # revealed: <module 'mypackage.submodule'>
# TODO: this would be nice to support
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `nested` might not have been imported"
reveal_type(mypackage.submodule.nested.X)  # revealed: Unknown
reveal_type(mypackage.nested)  # revealed: <module 'mypackage.submodule.nested'>
reveal_type(mypackage.nested.X)  # revealed: int
```

## Import of Nested Submodule in `__init__`

`import mypackage.submodule.nested` in an `__init__.pyi` does not re-export `mypackage.submodule` or
`mypackage.submodule.nested`.

### In Stub

`mypackage/__init__.pyi`:

```pyi
import mypackage.submodule.nested
```

`mypackage/submodule/__init__.pyi`:

```pyi
```

`mypackage/submodule/nested.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage

# error: [possibly-missing-submodule] "Submodule `submodule` might not have been imported"
reveal_type(mypackage.submodule)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `submodule` might not have been imported"
reveal_type(mypackage.submodule.nested)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `submodule` might not have been imported"
reveal_type(mypackage.submodule.nested.X)  # revealed: Unknown
```

### In Non-Stub

`import mypackage.submodule.nested` in an `__init__.py` does not define `mypackage.submodule` or
`mypackage.submodule.nested` outside the package.

`mypackage/__init__.py`:

```py
import mypackage.submodule.nested
```

`mypackage/submodule/__init__.py`:

```py
```

`mypackage/submodule/nested.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage

# TODO: this would be nice to support
# error: [possibly-missing-submodule] "Submodule `submodule` might not have been imported"
reveal_type(mypackage.submodule)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `submodule` might not have been imported"
reveal_type(mypackage.submodule.nested)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `submodule` might not have been imported"
reveal_type(mypackage.submodule.nested.X)  # revealed: Unknown
```

## Relative `from` Import of Direct Submodule in `__init__`, Mismatched Alias

Renaming the submodule to something else disables the `__init__.pyi` idiom.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from . import imported as imported_m
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage

# error: [possibly-missing-submodule] "Submodule `imported` might not have been imported"
reveal_type(mypackage.imported.X)  # revealed: Unknown
# error: [unresolved-attribute] "has no member `imported_m`"
reveal_type(mypackage.imported_m.X)  # revealed: Unknown
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from . import imported as imported_m
```

`mypackage/imported.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage

# TODO: this would be nice to support, as it works at runtime
# error: [possibly-missing-submodule] "Submodule `imported` might not have been imported"
reveal_type(mypackage.imported.X)  # revealed: Unknown
reveal_type(mypackage.imported_m.X)  # revealed: int
```

## Relative `from` Import of Direct Submodule in `__init__`, Matched Alias

The `__init__.pyi` idiom should definitely always work if the submodule is renamed to itself, as
this is the re-export idiom.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from . import imported as imported
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from . import imported as imported
```

`mypackage/imported.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
```

## Star Import Unaffected

Even if the `__init__` idiom is in effect, star imports do not pick it up. (This is an arbitrary
decision that mostly fell out of the implementation details and can be changed!)

### In Stub

`mypackage/__init__.pyi`:

```pyi
from . import imported

Z: int = 17
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
from mypackage import *

# TODO: this would be nice to support
# error: [unresolved-reference] "`imported` used when not defined"
reveal_type(imported.X)  # revealed: Unknown
reveal_type(Z)  # revealed: int
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from . import imported

Z: int = 17
```

`mypackage/imported.py`:

```py
X: int = 42
```

`main.py`:

```py
from mypackage import *

reveal_type(imported.X)  # revealed: int
reveal_type(Z)  # revealed: int
```

## `from` Import of Non-Submodule

A `from` import that imports a non-submodule isn't currently a special case here (various
proposed/tested approaches did treat this specially).

### In Stub

`mypackage/__init__.pyi`:

```pyi
from .imported import X
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from .imported import X
```

`mypackage/imported.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage

reveal_type(mypackage.imported.X)  # revealed: int
```

## `from` Import of Other Package's Submodule

`from mypackage import submodule` from outside the package is not modeled as a side-effect on
`mypackage`, even in the importing file (this could be changed!).

### In Stub

`mypackage/__init__.pyi`:

```pyi
```

`mypackage/imported.pyi`:

```pyi
X: int = 42
```

`main.py`:

```py
import mypackage
from mypackage import imported

reveal_type(imported.X)  # revealed: int

# TODO: this would be nice to support, but it's dangerous with available_submodule_attributes
# for details, see: https://github.com/astral-sh/ty/issues/1488
# error: [possibly-missing-submodule] "Submodule `imported` might not have been imported"
reveal_type(mypackage.imported.X)  # revealed: Unknown
```

### In Non-Stub

`mypackage/__init__.py`:

```py
```

`mypackage/imported.py`:

```py
X: int = 42
```

`main.py`:

```py
import mypackage
from mypackage import imported

reveal_type(imported.X)  # revealed: int

# TODO: this would be nice to support, as it works at runtime
# error: [possibly-missing-submodule] "Submodule `imported` might not have been imported"
reveal_type(mypackage.imported.X)  # revealed: Unknown
```

## `from` Import of Sibling Module

`from . import submodule` from a sibling module is not modeled as a side-effect on `mypackage` or a
re-export from `submodule`.

### In Stub

`mypackage/__init__.pyi`:

```pyi
```

`mypackage/imported.pyi`:

```pyi
from . import fails

X: int = 42
```

`mypackage/fails.pyi`:

```pyi
Y: int = 47
```

`main.py`:

```py
import mypackage
from mypackage import imported

reveal_type(imported.X)  # revealed: int
# error: [unresolved-attribute] "has no member `fails`"
reveal_type(imported.fails.Y)  # revealed: Unknown
# error: [possibly-missing-submodule] "Submodule `fails` might not have been imported"
reveal_type(mypackage.fails.Y)  # revealed: Unknown
```

### In Non-Stub

`mypackage/__init__.py`:

```py
```

`mypackage/imported.py`:

```py
from . import fails

X: int = 42
```

`mypackage/fails.py`:

```py
Y: int = 47
```

`main.py`:

```py
import mypackage
from mypackage import imported

reveal_type(imported.X)  # revealed: int
reveal_type(imported.fails.Y)  # revealed: int
# error: [possibly-missing-submodule] "Submodule `fails`"
reveal_type(mypackage.fails.Y)  # revealed: Unknown
```

## Fractal Re-export Nameclash Problems

This precise configuration of:

- a subpackage that defines a submodule with its own name
- that in turn defines a function/class with its own name
- and re-exporting that name through every layer using `from` imports and `__all__`

Can easily result in the typechecker getting "confused" and thinking imports of the name from the
top-level package are referring to the subpackage and not the function/class. This issue can be
found with the `lobpcg` function in `scipy.sparse.linalg`.

We avoid this by ensuring that the imported name (the right-hand `funcmod` in
`from .funcmod import funcmod`) overwrites the submodule attribute (the left-hand `funcmod`), as it
does at runtime.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from .funcmod import funcmod

__all__ = ["funcmod"]
```

`mypackage/funcmod/__init__.pyi`:

```pyi
from .funcmod import funcmod

__all__ = ["funcmod"]
```

`mypackage/funcmod/funcmod.pyi`:

```pyi
__all__ = ["funcmod"]

def funcmod(x: int) -> int: ...
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from .funcmod import funcmod

__all__ = ["funcmod"]
```

`mypackage/funcmod/__init__.py`:

```py
from .funcmod import funcmod

__all__ = ["funcmod"]
```

`mypackage/funcmod/funcmod.py`:

```py
__all__ = ["funcmod"]

def funcmod(x: int) -> int:
    return x
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```

## Re-export Nameclash Problems In Functions

`from` imports in an `__init__.py` at file scope should be visible to functions defined in the file:

`mypackage/__init__.py`:

```py
from .funcmod import funcmod

funcmod(1)

def run():
    funcmod(2)
```

`mypackage/funcmod.py`:

```py
def funcmod(x: int) -> int:
    return x
```

## Re-export Nameclash Problems After Conditional Raise

`from` imports in an `__init__.py` at file scope should overwrite the submodule attribute even when
earlier ambiguous control flow causes us to consider more bindings from nested functions.

`mypackage/__init__.py`:

```py
def should_raise() -> bool:
    return True

if should_raise():
    raise RuntimeError

from .funcmod import funcmod

reveal_type(funcmod)  # revealed: def funcmod(x: int) -> int

def run():
    reveal_type(funcmod)  # revealed: def funcmod(x: int) -> int
    funcmod(1)
```

`mypackage/funcmod.py`:

```py
def funcmod(x: int) -> int:
    return x
```

## Re-export Nameclash Problems In Try-Blocks

`from` imports in an `__init__.py` at file scope in a `try` block should be visible to functions
defined in the `try` block (regression test for a bug):

`mypackage/__init__.py`:

```py
try:
    from .funcmod import funcmod

    funcmod(1)

    def run():
        funcmod(2)

finally:
    x = 1
```

`mypackage/funcmod.py`:

```py
def funcmod(x: int) -> int:
    return x
```

## RHS `from` Imports In Functions

If a `from` import occurs in a function, the RHS symbols should only be visible in that function.

`mypackage/__init__.py`:

```py
def run1():
    from .funcmod import funcmod

    funcmod(1)

def run2():
    from .funcmod import funcmod

    funcmod(2)

def run3():
    # error: [unresolved-reference]
    funcmod(3)

# error: [unresolved-reference]
funcmod(4)
```

`mypackage/funcmod.py`:

```py
def funcmod(x: int) -> int:
    return x
```

## LHS `from` Imports In Functions

If a `from` import occurs in a function, we simply ignore its LHS effects to avoid modeling
execution-order-specific behaviour (and to discourage people writing code that has it).

`mypackage/__init__.py`:

```py
def run1():
    from .funcmod import other

    # TODO: this would be nice to support
    # error: [unresolved-reference]
    funcmod.funcmod(1)

def run2():
    from .funcmod import other

    # TODO: this would be nice to support
    # error: [unresolved-reference]
    funcmod.funcmod(2)

def run3():
    # error: [unresolved-reference]
    funcmod.funcmod(3)

# error: [unresolved-reference]
funcmod.funcmod(4)
```

`mypackage/funcmod.py`:

```py
other: int = 1

def funcmod(x: int) -> int:
    return x
```

## LHS `from` Imports Overwrite Locals

The LHS of a `from..import` introduces a local symbol that overwrites any local with the same name.
This reflects actual runtime behaviour, although we're kinda assuming it hasn't been imported
already.

`mypackage/__init__.py`:

```py
funcmod = 0
from .funcmod import funcmod

funcmod(1)
```

`mypackage/funcmod.py`:

```py
def funcmod(x: int) -> int:
    return x
```

## LHS `from` Imports Overwritten By Local Function

The LHS of a `from..import` introduces a local symbol that can be overwritten by defining a function
(or class) with the same name.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from .funcmod import other

def funcmod(x: int) -> int: ...
```

`mypackage/funcmod/__init__.pyi`:

```pyi
def other(int) -> int: ...
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from .funcmod import other

def funcmod(x: int) -> int:
    return x
```

`mypackage/funcmod/__init__.py`:

```py
def other(x: int) -> int:
    return x
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```

## LHS `from` Imports Overwritten By Local Assignment

The LHS of a `from..import` introduces a local symbol that can be overwritten by assigning to it.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from .funcmod import other

funcmod = other
```

`mypackage/funcmod/__init__.pyi`:

```pyi
def other(x: int) -> int: ...
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from .funcmod import other

funcmod = other
```

`mypackage/funcmod/__init__.py`:

```py
def other(x: int) -> int:
    return x
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```

## LHS `from` Imports Only Apply The First Time

The LHS of a `from..import` of a submodule introduces a local symbol only the first time it
introduces a direct submodule. The second time does nothing.

### In Stub

`mypackage/__init__.pyi`:

```pyi
from .funcmod import funcmod as funcmod
from .funcmod import other
```

`mypackage/funcmod/__init__.pyi`:

```pyi
def other(x: int) -> int: ...
def funcmod(x: int) -> int: ...
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```

### In Non-Stub

`mypackage/__init__.py`:

```py
from .funcmod import funcmod
from .funcmod import other
```

`mypackage/funcmod/__init__.py`:

```py
def other(x: int) -> int:
    return x

def funcmod(x: int) -> int:
    return x
```

`main.py`:

```py
from mypackage import funcmod

x = funcmod(1)
```
