[PythonKit] Added function to explicitly load the Python load

Author: pvieito

PythonKit/PythonLibrary.swift

@@ -28,9 +28,22 @@ import WinSDK
 //===----------------------------------------------------------------------===//
 
 public struct PythonLibrary {
+    public enum Error: Swift.Error, Equatable, CustomStringConvertible {
+        case pythonLibraryNotFound
+        
+        public var description: String {
+            switch self {
+            case .pythonLibraryNotFound:
+                return """
+                    Python library not found. Set the \(Environment.library.key) \
+                    environment variable with the path to a Python library.
+                    """
+            }
+        }
+    }
+    
     private static let pythonInitializeSymbolName = "Py_Initialize"
     private static let pythonLegacySymbolName = "PyString_AsString"
-    private static var isPythonLibraryLoaded = false
 
     #if canImport(Darwin)
     private static let defaultLibraryHandle = UnsafeMutableRawPointer(bitPattern: -2)  // RTLD_DEFAULT
@@ -40,21 +53,28 @@ public struct PythonLibrary {
     private static let defaultLibraryHandle: UnsafeMutableRawPointer? = nil  // Unsupported
     #endif
 
-    private static let pythonLibraryHandle: UnsafeMutableRawPointer? = {
-        let pythonLibraryHandle = Self.loadPythonLibrary()
-        guard Self.isPythonLibraryLoaded(at: pythonLibraryHandle) else {
-            fatalError("""
-                Python library not found. Set the \(Environment.library.key) \
-                environment variable with the path to a Python library.
-                """)
+    private static var isPythonLibraryLoaded = false
+    private static var _pythonLibraryHandle: UnsafeMutableRawPointer?
+    private static var pythonLibraryHandle: UnsafeMutableRawPointer? {
+        try! PythonLibrary.loadLibrary()
+        return self._pythonLibraryHandle
+    }
+    
+    /// Tries to load the Python library, will throw an error if no compatible library is found.
+    public static func loadLibrary() throws {
+        guard !self.isPythonLibraryLoaded else { return }
+        let pythonLibraryHandle = self.loadPythonLibrary()
+        guard self.isPythonLibraryLoaded(at: pythonLibraryHandle) else {
+            throw Error.pythonLibraryNotFound
         }
-        Self.isPythonLibraryLoaded = true
-        return pythonLibraryHandle
-    }()
+        self.isPythonLibraryLoaded = true
+        self._pythonLibraryHandle = pythonLibraryHandle
+    }
+    
     private static let isLegacyPython: Bool = {
-        let isLegacyPython = Self.loadSymbol(Self.pythonLibraryHandle, Self.pythonLegacySymbolName) != nil
+        let isLegacyPython = PythonLibrary.loadSymbol(PythonLibrary.pythonLibraryHandle, PythonLibrary.pythonLegacySymbolName) != nil
         if isLegacyPython {
-            Self.log("Loaded legacy Python library, using legacy symbols...")
+            PythonLibrary.log("Loaded legacy Python library, using legacy symbols...")
         }
         return isLegacyPython
     }()
@@ -194,7 +214,7 @@ extension PythonLibrary {
     }
 }
 
-// Methods of `PythonLibrary` required to set a given Python version.
+// Methods of `PythonLibrary` required to set a given Python version or library path.
 extension PythonLibrary {
     private static func enforceNonLoadedPythonLibrary(function: String = #function) {
         precondition(!self.isPythonLibraryLoaded, """