[util] add 'std' object to global eval namespace (#6330)

allows accessing standard library modules (and other external modules)
in a more straightforward manner than '__import__(...)'

* std.os.getcwd()
* std["os"].getcwd()

gallery_dl/util.py

@@ -532,6 +532,24 @@ class HTTPBasicAuth():
         return request
 
 
+class ModuleProxy():
+    __slots__ = ()
+
+    def __getitem__(self, key, modules=sys.modules):
+        try:
+            return modules[key]
+        except KeyError:
+            pass
+        try:
+            __import__(key)
+        except ImportError:
+            modules[key] = NONE
+            return NONE
+        return modules[key]
+
+    __getattr__ = __getitem__
+
+
 class LazyPrompt():
     __slots__ = ()
 
@@ -540,6 +558,7 @@ class LazyPrompt():
 
 
 class NullContext():
+    __slots__ = ()
 
     def __enter__(self):
         return None
@@ -646,6 +665,7 @@ GLOBALS = {
     "restart"  : raises(exception.RestartExtraction),
     "hash_sha1": sha1,
     "hash_md5" : md5,
+    "std"      : ModuleProxy(),
     "re"       : re,
 }
 

test/test_util.py

@@ -830,6 +830,34 @@ def hash(value):
             i += 1
         self.assertEqual(i, 0)
 
+    def test_module_proxy(self):
+        proxy = util.ModuleProxy()
+
+        self.assertIs(proxy.os, os)
+        self.assertIs(proxy.os.path, os.path)
+        self.assertIs(proxy["os"], os)
+        self.assertIs(proxy["os.path"], os.path)
+        self.assertIs(proxy["os"].path, os.path)
+
+        self.assertIs(proxy.abcdefghi, util.NONE)
+        self.assertIs(proxy["abcdefghi"], util.NONE)
+        self.assertIs(proxy["abc.def.ghi"], util.NONE)
+        self.assertIs(proxy["os.path2"], util.NONE)
+
+    def test_null_context(self):
+        with util.NullContext():
+            pass
+
+        with util.NullContext() as ctx:
+            self.assertIs(ctx, None)
+
+        try:
+            with util.NullContext() as ctx:
+                exc_orig = ValueError()
+                raise exc_orig
+        except ValueError as exc:
+            self.assertIs(exc, exc_orig)
+
 
 class TestExtractor():
     category = "test_category"