Add importing-resolve function to dynamically import Python names (#1072

)

Closes #1065 
Closes #1070

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
  * Added functions to `basilisp.test` for using and combining test fixtures (#980)
+ * Added the `importing-resolve` function for dynamically importing and resolving a Python name (#1065, #1070)
 
 ### Fixed
  * Fix a bug where the reader was double counting the CRLF newline seq in metadata (#1063)

src/basilisp/core.lpy

@@ -4886,7 +4886,13 @@
   if the ``import`` itself occurs within a function or method.
 
   Use of ``import`` directly is discouraged in favor of the ``:import`` directive in
-  the :lpy:fn:`ns` macro."
+  the :lpy:fn:`ns` macro.
+
+  Importing and attempting to reference an import within a top level form other than
+  a :lpy:form:`do` is a compile-time error because the compiler cannot verify that
+  the imported name exists. In cases where you may need to import and reference an
+  imported symbol within a single top-level form, you can use
+  :lpy:fn:`importing-resolve`."
   [& modules]
   ;; We need to use eval here, because there could have been an ns
   ;; switch inside an eval earlier, and although *ns* is correctly set
@@ -5065,6 +5071,12 @@
   Use of ``require`` directly is discouraged in favor of the ``:require`` directive in
   the :lpy:fn:`ns` macro.
 
+  Requiring and attempting to reference a required namespace within a top level form
+  other than a :lpy:form:`do` is a compile-time error because the compiler cannot
+  verify that the required name exists. In cases where you may need to require and
+  reference a required symbol within a single top-level form, you can use
+  :lpy:fn:`requiring-resolve`.
+
   .. warning::
 
      Reloading namespaces has many of the same limitations described for
@@ -5265,6 +5277,23 @@
        ~uses
        ~@imports)))
 
+(defn importing-resolve
+  "Resolve the namespaced symbol ``sym`` as a Python module member. If resolution fails,
+  attempts to import ``sym`` 's namespace (as by :lpy:fn:`import`\\) before resolving
+  again."
+  [sym]
+  (if (qualified-symbol? sym)
+    (let [mod-name   (basilisp.lang.util/munge (namespace sym))
+          obj-name   (basilisp.lang.util/munge (name sym))
+          py-resolve #(some-> (get sys/modules mod-name)
+                              (python/getattr obj-name nil))]
+      (or (py-resolve)
+          (do (eval `(import* ~(symbol (namespace sym))) *ns*)
+              (py-resolve))))
+    (throw
+     (ex-info "Cannot resolve an unqualified symbol"
+              {:sym sym}))))
+
 (defn requiring-resolve
   "Resolve the namespaced symbol ``sym`` as by :lpy:fn:`resolve`\\. If resolution fails,
   attempts to require ``sym`` 's namespace (as by :lpy:fn:`require`\\) before resolving

tests/basilisp/test_core_fns.lpy

@@ -1748,6 +1748,18 @@
   ;; a valid symbol in that namespace
   (is (= (resolve 'basilisp.shell/sh) (requiring-resolve 'basilisp.shell/sh))))
 
+(deftest importing-resolve-test
+  (is (thrown? basilisp.lang.exception/ExceptionInfo (importing-resolve 'a-symbol)))
+  (is (thrown? basilisp.lang.exception/ExceptionInfo (importing-resolve :a-keyword)))
+  (is (thrown? basilisp.lang.exception/ExceptionInfo (importing-resolve :ns.qualified/kw)))
+  (is (thrown? python/ImportError (importing-resolve 'not.a.real.ns/sym)))
+  (is (nil? (importing-resolve 'logging/fakeGetLogger)))
+  ;; this check should succeed (in spite of the missing top-level require) because
+  ;; the previous assertion will import 'basilisp.shell even though 'sholl is not
+  ;; a valid symbol in that namespace
+  (is (= (python/getattr (get sys/modules "logging") "getLogger")
+         (importing-resolve 'logging/getLogger))))
+
 (deftest ns-resolve-test
   (is (= #'basilisp.core/apply (ns-resolve *ns* 'apply)))
   (is (nil? (ns-resolve *ns* 'xyz)))