Make PYO3 dependency option for origen_metal crate

rust/origen_metal/Cargo.toml

@@ -13,13 +13,18 @@ categories = ["embedded", "science"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[features]
+# Enables the Python API, when not enabled (the default) origen_metal is a pure Rust library that
+# can only be used from Rust
+python = ["dep:pyo3"]
+
 [dependencies]
 lazy_static = "1"
 time = "0.1"
 termcolor = "1"
 regex = "1"
 tempfile = "3"
-pyo3 = {version = "0.19.1", features = ["extension-module"]}
+pyo3 = {version = "0.19.1", features = ["extension-module"], optional = true}
 serde = {version = "1.0", features = ["derive"]}
 toml = "0.5"
 toml_edit = "0.21.0"

rust/origen_metal/src/error.rs

@@ -28,12 +28,14 @@ impl BaseError for Error {
 
 // To add a conversion from other type of errors
 
+#[cfg(feature = "python")]
 impl std::convert::From<Error> for pyo3::PyErr {
     fn from(err: Error) -> pyo3::PyErr {
         pyo3::exceptions::PyRuntimeError::new_err(err.to_string())
     }
 }
 
+#[cfg(feature = "python")]
 impl std::convert::From<pyo3::PyErr> for Error {
     fn from(err: pyo3::PyErr) -> Self {
         pyo3::Python::with_gil(|py| {

rust/pyapi/Cargo.toml

@@ -14,7 +14,7 @@ crate-type = ["cdylib"]
 
 [dependencies]
 origen = { path = "../origen" }
-origen_metal = { path = "../origen_metal" }
+origen_metal = { path = "../origen_metal", features = ["python"] }
 pyapi_metal = { path = "../pyapi_metal", package = "pyapi_metal" }
 num-bigint = "0.4.0"
 num-traits = "0.2.0"

rust/pyapi_metal/Cargo.toml

@@ -11,7 +11,7 @@ name = "origen_metal"
 crate-type = ["cdylib", "lib"]
 
 [dependencies]
-origen_metal = { path = "../origen_metal" }
+origen_metal = { path = "../origen_metal", features = ["python"] }
 indexmap = "1.3.0"
 num-bigint = "0.4.0"