Added exception handling to account for non-standard Python environments in which readline is not loaded dynamically from a shared library file

Author: kmvanbrunt

CHANGELOG.md

@@ -1,6 +1,9 @@
 ## 0.9.21 (TBD, 2019)
 * Bug Fixes
     * Fixed bug where pipe processes were not being stopped by Ctrl-C
+    * Added exception handling to account for non-standard Python environments in which readline is not loaded
+     dynamically from a shared library file
+    
 * Enhancements
     * Added `read_input()` function that is used to read from stdin. Unlike the Python built-in `input()`, it also has
     an argument to disable tab completion while input is being entered.

cmd2/cmd2.py

@@ -52,13 +52,10 @@
 from .decorators import with_argparser
 from .history import History, HistoryItem
 from .parsing import StatementParser, Statement, Macro, MacroArg, shlex_split
-from .rl_utils import rl_type, RlType, rl_get_point, rl_set_prompt, vt100_support, rl_make_safe_prompt
+from .rl_utils import rl_type, RlType, rl_get_point, rl_set_prompt, vt100_support, rl_make_safe_prompt, rl_warning
 
 # Set up readline
 if rl_type == RlType.NONE:  # pragma: no cover
-    rl_warning = ("Readline features including tab completion have been disabled since no\n"
-                  "supported version of readline was found. To resolve this, install pyreadline\n"
-                  "on Windows or gnureadline on Mac.\n\n")
     sys.stderr.write(ansi.style_warning(rl_warning))
 else:
     from .rl_utils import rl_force_redisplay, readline

cmd2/rl_utils.py

@@ -32,6 +32,9 @@ class RlType(Enum):
 # Tells if the terminal we are running in supports vt100 control characters
 vt100_support = False
 
+# Explanation for why readline wasn't loaded
+_rl_warn_reason = ''
+
 # The order of this check matters since importing pyreadline will also show readline in the modules list
 if 'pyreadline' in sys.modules:
     rl_type = RlType.PYREADLINE
@@ -113,15 +116,27 @@ def pyreadline_remove_history_item(pos: int) -> None:
 elif 'gnureadline' in sys.modules or 'readline' in sys.modules:
     # We don't support libedit
     if 'libedit' not in readline.__doc__:
-        rl_type = RlType.GNU
-
-        # Load the readline lib so we can access members of it
-        import ctypes
-        readline_lib = ctypes.CDLL(readline.__file__)
-
-        # Check if we are running in a terminal
-        if sys.stdout.isatty():
-            vt100_support = True
+        try:
+            # Load the readline lib so we can access members of it
+            import ctypes
+            readline_lib = ctypes.CDLL(readline.__file__)
+        except AttributeError:
+            _rl_warn_reason = ("this application is running in a non-standard Python environment in\n"
+                               "which readline is not loaded dynamically from a shared library file")
+        else:
+            rl_type = RlType.GNU
+            if sys.stdout.isatty():
+                vt100_support = True
+
+# Check if readline was loaded
+if rl_type == RlType.NONE:
+    if not _rl_warn_reason:
+        _rl_warn_reason = ("no supported version of readline was found. To resolve this, install\n"
+                           "pyreadline on Windows or gnureadline on Mac.")
+    rl_warning = ("Readline features including tab completion have been disabled because\n" +
+                  _rl_warn_reason + '\n\n')
+else:
+    rl_warning = ''
 
 
 # noinspection PyProtectedMember,PyUnresolvedReferences