add #[pyo3(signature = (...))] attribute

examples/decorator/src/lib.rs

@@ -36,7 +36,7 @@ impl PyCounter {
         self.count.get()
     }
 
-    #[args(args = "*", kwargs = "**")]
+    #[pyo3(signature = (*args, **kwargs))]
     fn __call__(
         &self,
         py: Python<'_>,

guide/src/class.md

@@ -632,9 +632,9 @@ impl MyClass {
 
 ## Method arguments
 
-Similar to `#[pyfunction]`, the `#[args]` attribute can be used to specify the way that `#[pymethods]` accept arguments. Consult the documentation for [`function signatures`](./function/signature.md) to see the parameters this attribute accepts.
+Similar to `#[pyfunction]`, the `#[pyo3(signature = (...))]` attribute can be used to specify the way that `#[pymethods]` accept arguments. Consult the documentation for [`function signatures`](./function/signature.md) to see the parameters this attribute accepts.
 
-The following example defines a class `MyClass` with a method `method`. This method has an `#[args]` attribute which sets default values for `num` and `name`, and indicates that `py_args` should collect all extra positional arguments and `py_kwargs` all extra keyword arguments:
+The following example defines a class `MyClass` with a method `method`. This method has a signature which sets default values for `num` and `name`, and indicates that `py_args` should collect all extra positional arguments and `py_kwargs` all extra keyword arguments:
 
 ```rust
 # use pyo3::prelude::*;
@@ -647,29 +647,24 @@ use pyo3::types::{PyDict, PyTuple};
 #[pymethods]
 impl MyClass {
     #[new]
-    #[args(num = "-1")]
+    #[pyo3(signature = (num=-1))]
     fn new(num: i32) -> Self {
         MyClass { num }
     }
 
-    #[args(
-        num = "10",
-        py_args = "*",
-        name = "\"Hello\"",
-        py_kwargs = "**"
-    )]
+    #[pyo3(signature = (num=10, *py_args, name="Hello", **py_kwargs))]
     fn method(
         &mut self,
         num: i32,
-        name: &str,
         py_args: &PyTuple,
+        name: &str,
         py_kwargs: Option<&PyDict>,
     ) -> String {
         let num_before = self.num;
         self.num = num;
         format!(
-            "py_args={:?}, py_kwargs={:?}, name={}, num={} num_before={}",
-            py_args, py_kwargs, name, self.num, num_before,
+            "num={} (was previously={}), py_args={:?}, name={}, py_kwargs={:?} ",
+            num, num_before, py_args, name, py_kwargs,
         )
     }
 }

guide/src/class/call.md

@@ -71,7 +71,7 @@ def Counter(wraps):
 A [previous implementation] used a normal `u64`, which meant it required a `&mut self` receiver to update the count:
 
 ```rust,ignore
-#[args(args = "*", kwargs = "**")]
+#[pyo3(signature = (*args, **kwargs))]
 fn __call__(&mut self, py: Python<'_>, args: &PyTuple, kwargs: Option<&PyDict>) -> PyResult<Py<PyAny>> {
     self.count += 1;
     let name = self.wraps.getattr(py, "__name__")?;
@@ -98,7 +98,7 @@ As a result, something innocent like this will raise an exception:
 def say_hello():
     if say_hello.count < 2:
         print(f"hello from decorator")
-        
+
 say_hello()
 # RuntimeError: Already borrowed
 ```
@@ -113,4 +113,4 @@ This shows the dangers of running arbitrary Python code - note that "running arb
 This is especially important if you are writing unsafe code; Python code must never be able to cause undefined behavior. You must ensure that your Rust code is in a consistent state before doing any of the above things.
 
 [previous implementation]: https://github.com/PyO3/pyo3/discussions/2598 "Thread Safe Decorator <Help Wanted> · Discussion #2598 · PyO3/pyo3"
-[`Cell`]: https://doc.rust-lang.org/std/cell/struct.Cell.html "Cell in std::cell - Rust"
+[`Cell`]: https://doc.rust-lang.org/std/cell/struct.Cell.html "Cell in std::cell - Rust"

guide/src/function.md

@@ -23,6 +23,7 @@ This chapter of the guide explains full usage of the `#[pyfunction]` attribute.
 
 - [Function options](#function-options)
   - [`#[pyo3(name = "...")]`](#name)
+  - [`#[pyo3(signature = (...))]`](#signature)
   - [`#[pyo3(text_signature = "...")]`](#text_signature)
   - [`#[pyo3(pass_module)]`](#pass_module)
 - [Per-argument options](#per-argument-options)
@@ -64,6 +65,10 @@ The `#[pyo3]` attribute can be used to modify properties of the generated Python
     # });
     ```
 
+  - <a name="signature"></a> `#[pyo3(signature = (...))]`
+
+    Defines the function signature in Python. See [Function Signatures](./function/signature.md).
+
   - <a name="text_signature"></a> `#[pyo3(text_signature = "...")]`
 
     Sets the function signature visible in Python tooling (such as via [`inspect.signature`]).

guide/src/function/signature.md

@@ -2,9 +2,121 @@
 
 The `#[pyfunction]` attribute also accepts parameters to control how the generated Python function accepts arguments. Just like in Python, arguments can be positional-only, keyword-only, or accept either. `*args` lists and `**kwargs` dicts can also be accepted. These parameters also work for `#[pymethods]` which will be introduced in the [Python Classes](../class.md) section of the guide.
 
-Like Python, by default PyO3 accepts all arguments as either positional or keyword arguments. The extra arguments to `#[pyfunction]` modify this behaviour. For example, below is a function that accepts arbitrary keyword arguments (`**kwargs` in Python syntax) and returns the number that was passed:
+Like Python, by default PyO3 accepts all arguments as either positional or keyword arguments. There are two ways to modify this behaviour:
+  - The `#[pyo3(signature = (...))]` option which allows writing a signature in Python syntax.
+  - Extra arguments directly to `#[pyfunction]`. (See deprecated form)
+
+## Using `#[pyo3(signature = (...))]`
+
+For example, below is a function that accepts arbitrary keyword arguments (`**kwargs` in Python syntax) and returns the number that was passed:
+
+```rust
+use pyo3::prelude::*;
+use pyo3::types::PyDict;
+
+#[pyfunction]
+#[pyo3(signature = (**kwds))]
+fn num_kwds(kwds: Option<&PyDict>) -> usize {
+    kwds.map_or(0, |dict| dict.len())
+}
+
+#[pymodule]
+fn module_with_functions(py: Python<'_>, m: &PyModule) -> PyResult<()> {
+    m.add_function(wrap_pyfunction!(num_kwds, m)?).unwrap();
+    Ok(())
+}
+```
+
+Just like in Python, the following constructs can be part of the signature::
+
+ * `/`: positional-only arguments separator, each parameter defined before `/` is a positional-only parameter.
+ * `*`: var arguments separator, each parameter defined after `*` is a keyword-only parameter.
+ * `*args`: "args" is var args. Type of the `args` parameter has to be `&PyTuple`.
+ * `**kwargs`: "kwargs" receives keyword arguments. The type of the `kwargs` parameter has to be `Option<&PyDict>`.
+ * `arg=Value`: arguments with default value.
+   If the `arg` argument is defined after var arguments, it is treated as a keyword-only argument.
+   Note that `Value` has to be valid rust code, PyO3 just inserts it into the generated
+   code unmodified.
+
+Example:
+```rust
+# use pyo3::prelude::*;
+use pyo3::types::{PyDict, PyTuple};
+#
+# #[pyclass]
+# struct MyClass {
+#     num: i32,
+# }
+#[pymethods]
+impl MyClass {
+    #[new]
+    #[pyo3(signature = (num=-1))]
+    fn new(num: i32) -> Self {
+        MyClass { num }
+    }
+
+    #[pyo3(signature = (num=10, *py_args, name="Hello", **py_kwargs))]
+    fn method(
+        &mut self,
+        num: i32,
+        py_args: &PyTuple,
+        name: &str,
+        py_kwargs: Option<&PyDict>,
+    ) -> String {
+        let num_before = self.num;
+        self.num = num;
+        format!(
+            "num={} (was previously={}), py_args={:?}, name={}, py_kwargs={:?} ",
+            num, num_before, py_args, name, py_kwargs,
+        )
+    }
+
+    fn make_change(&mut self, num: i32) -> PyResult<String> {
+        self.num = num;
+        Ok(format!("num={}", self.num))
+    }
+}
+```
+N.B. the position of the `/` and `*` arguments (if included) control the system of handling positional and keyword arguments. In Python:
+```python
+import mymodule
+
+mc = mymodule.MyClass()
+print(mc.method(44, False, "World", 666, x=44, y=55))
+print(mc.method(num=-1, name="World"))
+print(mc.make_change(44, False))
+```
+Produces output:
+```text
+py_args=('World', 666), py_kwargs=Some({'x': 44, 'y': 55}), name=Hello, num=44
+py_args=(), py_kwargs=None, name=World, num=-1
+num=44
+num=-1
+```
+
+> Note: for keywords like `struct`, to use it as a function argument, use "raw ident" syntax `r#struct` in both the signature and the function definition:
+>
+> ```rust
+> # #![allow(dead_code)]
+> # use pyo3::prelude::*;
+> #[pyfunction(signature = (r#struct = "foo"))]
+> fn function_with_keyword(r#struct: &str) {
+> #     let _ = r#struct;
+>     /* ... */
+> }
+> ```
+
+## Deprecated form
+
+The `#[pyfunction]` macro can take the argument specification directly, but this method is deprecated in PyO3 0.18 because the `#[pyo3(signature)]` option offers a simpler syntax and better validation.
+
+The `#[pymethods]` macro has an `#[args]` attribute which accepts the deprecated form.
+
+Below are the same examples as above which using the deprecated syntax:
 
 ```rust
+# #![allow(deprecated)]
+
 use pyo3::prelude::*;
 use pyo3::types::PyDict;
 
@@ -38,6 +150,7 @@ The following parameters can be passed to the `#[pyfunction]` attribute:
 
 Example:
 ```rust
+# #![allow(deprecated)]
 # use pyo3::prelude::*;
 use pyo3::types::{PyDict, PyTuple};
 #
@@ -62,15 +175,16 @@ impl MyClass {
     fn method(
         &mut self,
         num: i32,
-        name: &str,
         py_args: &PyTuple,
+        name: &str,
         py_kwargs: Option<&PyDict>,
-    ) -> PyResult<String> {
+    ) -> String {
+        let num_before = self.num;
         self.num = num;
-        Ok(format!(
-            "py_args={:?}, py_kwargs={:?}, name={}, num={}",
-            py_args, py_kwargs, name, self.num
-        ))
+        format!(
+            "num={} (was previously={}), py_args={:?}, name={}, py_kwargs={:?} ",
+            num, num_before, py_args, name, py_kwargs,
+        )
     }
 
     fn make_change(&mut self, num: i32) -> PyResult<String> {
@@ -79,19 +193,3 @@ impl MyClass {
     }
 }
 ```
-N.B. the position of the `"/"` and `"*"` arguments (if included) control the system of handling positional and keyword arguments. In Python:
-```python
-import mymodule
-
-mc = mymodule.MyClass()
-print(mc.method(44, False, "World", 666, x=44, y=55))
-print(mc.method(num=-1, name="World"))
-print(mc.make_change(44, False))
-```
-Produces output:
-```text
-py_args=('World', 666), py_kwargs=Some({'x': 44, 'y': 55}), name=Hello, num=44
-py_args=(), py_kwargs=None, name=World, num=-1
-num=44
-num=-1
-```

newsfragments/2702.added.md

@@ -0,0 +1 @@
+Add `#[pyo3(signature = (...))]` option for `#[pyfunction]` and `#[pymethods]`.

newsfragments/2702.changed.md

@@ -0,0 +1 @@
+Deprecate `#[args]` attribute and passing "args" specification directly to `#[pyfunction]` in favour of the new `#[pyo3(signature = (...))]` option.

pyo3-macros-backend/src/attributes.rs

@@ -9,6 +9,7 @@ use syn::{
 };
 
 pub mod kw {
+    syn::custom_keyword!(args);
     syn::custom_keyword!(annotation);
     syn::custom_keyword!(attribute);
     syn::custom_keyword!(dict);
@@ -42,7 +43,7 @@ pub struct KeywordAttribute<K, V> {
 }
 
 /// A helper type which parses the inner type via a literal string
-/// e.g. LitStrValue<Path> -> parses "some::path" in quotes.
+/// e.g. `LitStrValue<Path>` -> parses "some::path" in quotes.
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct LitStrValue<T>(pub T);
 

pyo3-macros-backend/src/deprecations.rs

@@ -1,14 +1,20 @@
 use proc_macro2::{Span, TokenStream};
 use quote::{quote_spanned, ToTokens};
 
+// Clippy complains all these variants have the same prefix "Py"...
+#[allow(clippy::enum_variant_names)]
 pub enum Deprecation {
     PyClassGcOption,
+    PyFunctionArguments,
+    PyMethodArgsAttribute,
 }
 
 impl Deprecation {
     fn ident(&self, span: Span) -> syn::Ident {
         let string = match self {
             Deprecation::PyClassGcOption => "PYCLASS_GC_OPTION",
+            Deprecation::PyFunctionArguments => "PYFUNCTION_ARGUMENTS",
+            Deprecation::PyMethodArgsAttribute => "PYMETHODS_ARGS_ATTRIBUTE",
         };
         syn::Ident::new(string, span)
     }