support accessing environment variables in format strings (#1968)

{_env[HOME]} to get the value of $HOME
every other format string feature is supported as well

gallery_dl/formatter.py

@@ -8,13 +8,15 @@
 
 """String formatters"""
 
+import os
 import json
 import string
 import _string
 import operator
 from . import text, util
 
 _CACHE = {}
+_GLOBALS = {"_env": os.environ}
 _CONVERSIONS = None
 
 
@@ -125,6 +127,8 @@ def _field_access(self, field_name, format_spec, conversion):
             ], fmt)
         else:
             key, funcs = parse_field_name(field_name)
+            if key in _GLOBALS:
+                return self._apply_globals(_GLOBALS[key], funcs, fmt)
             if funcs:
                 return self._apply(key, funcs, fmt)
             return self._apply_simple(key, fmt)
@@ -140,6 +144,17 @@ def wrap(kwdict):
             return fmt(obj)
         return wrap
 
+    def _apply_globals(self, gobj, funcs, fmt):
+        def wrap(_):
+            try:
+                obj = gobj
+                for func in funcs:
+                    obj = func(obj)
+            except Exception:
+                obj = self.default
+            return fmt(obj)
+        return wrap
+
     def _apply_simple(self, key, fmt):
         def wrap(kwdict):
             return fmt(kwdict[key] if key in kwdict else self.default)
@@ -149,7 +164,7 @@ def _apply_list(self, lst, fmt):
         def wrap(kwdict):
             for key, funcs in lst:
                 try:
-                    obj = kwdict[key]
+                    obj = _GLOBALS[key] if key in _GLOBALS else kwdict[key]
                     for func in funcs:
                         obj = func(obj)
                     if obj:

test/test_formatter.py

@@ -173,6 +173,13 @@ def test_chain_special(self):
         self._run_test("{d[a]:?</>/L1/too long/}", "<too long>")
         self._run_test("{d[c]:?</>/L5/too long/}", "")
 
+    def test_environ(self):
+        os.environ["FORMATTER_TEST"] = value = self.kwdict["a"]
+
+        self._run_test("{_env[FORMATTER_TEST]}"  , value)
+        self._run_test("{_env[FORMATTER_TEST]!l}", value.lower())
+        self._run_test("{z|_env[FORMATTER_TEST]}", value)
+
     def _run_test(self, format_string, result, default=None):
         fmt = formatter.parse(format_string, default)
         output = fmt.format_map(self.kwdict)