Merge pull request #818 from python-cmd2/readline_warning

Readline warning

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/argparse_custom.py

@@ -4,7 +4,8 @@
 class called Cmd2ArgumentParser which improves error and help output over normal argparse. All cmd2 code uses
 this parser and it is recommended that developers of cmd2-based apps either use it or write their own parser
 that inherits from it. This will give a consistent look-and-feel between the help/error output of built-in
-cmd2 commands and the app-specific commands.
+cmd2 commands and the app-specific commands. If you wish to override the parser used by cmd2's built-in
+commands, see override_parser.py example.
 
 Since the new capabilities are added by patching at the argparse API level, they are available whether or not
 Cmd2ArgumentParser is used. However, the help and error output of Cmd2ArgumentParser is customized to notate

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 +116,26 @@ 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:  # pragma: no cover
+            _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
+            vt100_support = sys.stdout.isatty()
+
+# Check if readline was loaded
+if rl_type == RlType.NONE:  # pragma: no cover
+    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