feat: Ex command ":py=" evaluate and print expression #548

The Ex command `:py`, `:python`, :`py3`, etc. can evaluate the line
as an expression rather than a statement if the line starts with `=`,
just like `:lua=`.

`:py= <expr>` is equivalent as `:py print(<expr>)`.

```vim
:py3= sys.version_info[:3]
:python3 =pynvim.__version__
```

pynvim/msgpack_rpc/session.py

@@ -243,12 +243,12 @@ def handler():
                       + 'sending %s as response', gr, rv)
                 response.send(rv)
             except ErrorResponse as err:
-                warn("error response from request '%s %s': %s", name,
-                     args, format_exc())
+                debug("error response from request '%s %s': %s",
+                      name, args, format_exc())
                 response.send(err.args[0], error=True)
             except Exception as err:
-                warn("error caught while processing request '%s %s': %s", name,
-                     args, format_exc())
+                warn("error caught while processing request '%s %s': %s",
+                     name, args, format_exc())
                 response.send(repr(err) + "\n" + format_exc(5), error=True)
             debug('greenlet %s is now dying...', gr)
 

pynvim/plugin/script_host.py

@@ -81,10 +81,19 @@ def teardown(self):
     def python_execute(self, script, range_start, range_stop):
         """Handle the `python` ex command."""
         self._set_current_range(range_start, range_stop)
+
+        if script.startswith('='):
+            # Handle ":py= ...". Evaluate as an expression and print.
+            # (note: a valid python statement can't start with "=")
+            expr = script[1:]
+            print(self.python_eval(expr))
+            return
+
         try:
+            # pylint: disable-next=exec-used
             exec(script, self.module.__dict__)
-        except Exception:
-            raise ErrorResponse(format_exc_skip(1))
+        except Exception as exc:
+            raise ErrorResponse(format_exc_skip(1)) from exc
 
     @rpc_export('python_execute_file', sync=True)
     def python_execute_file(self, file_path, range_start, range_stop):
@@ -93,9 +102,10 @@ def python_execute_file(self, file_path, range_start, range_stop):
         with open(file_path, 'rb') as f:
             script = compile(f.read(), file_path, 'exec')
             try:
+                # pylint: disable-next=exec-used
                 exec(script, self.module.__dict__)
-            except Exception:
-                raise ErrorResponse(format_exc_skip(1))
+            except Exception as exc:
+                raise ErrorResponse(format_exc_skip(1)) from exc
 
     @rpc_export('python_do_range', sync=True)
     def python_do_range(self, start, stop, code):
@@ -154,7 +164,11 @@ def python_do_range(self, start, stop, code):
     @rpc_export('python_eval', sync=True)
     def python_eval(self, expr):
         """Handle the `pyeval` vim function."""
-        return eval(expr, self.module.__dict__)
+        try:
+            # pylint: disable-next=eval-used
+            return eval(expr, self.module.__dict__)
+        except Exception as exc:
+            raise ErrorResponse(format_exc_skip(1)) from exc
 
     @rpc_export('python_chdir', sync=False)
     def python_chdir(self, cwd):

test/test_vim.py

@@ -1,11 +1,12 @@
 import os
 import sys
 import tempfile
+import textwrap
 from pathlib import Path
 
 import pytest
 
-from pynvim.api import Nvim
+from pynvim.api import Nvim, NvimError
 
 
 def source(vim: Nvim, code: str) -> None:
@@ -40,6 +41,10 @@ def test_command(vim: Nvim) -> None:
 def test_command_output(vim: Nvim) -> None:
     assert vim.command_output('echo "test"') == 'test'
 
+    # can capture multi-line outputs
+    vim.command("let g:multiline_string = join(['foo', 'bar'], nr2char(10))")
+    assert vim.command_output('echo g:multiline_string') == "foo\nbar"
+
 
 def test_command_error(vim: Nvim) -> None:
     with pytest.raises(vim.error) as excinfo:
@@ -213,6 +218,35 @@ def test_python3(vim: Nvim) -> None:
     assert 1 == vim.eval('has("python3")')
 
 
+def test_python3_ex_eval(vim: Nvim) -> None:
+    assert '42' == vim.command_output('python3 =42')
+    assert '42' == vim.command_output('python3 =   42     ')
+    assert '42' == vim.command_output('py3=    42     ')
+    assert '42' == vim.command_output('py=42')
+
+    # On syntax error or evaluation error, stacktrace information is printed
+    # Note: the pynvim API command_output() throws an exception on error
+    # because the Ex command :python will throw (wrapped with provider#python3#Call)
+    with pytest.raises(NvimError) as excinfo:
+        vim.command('py3= 1/0')
+    assert textwrap.dedent('''\
+        Traceback (most recent call last):
+          File "<string>", line 1, in <module>
+        ZeroDivisionError: division by zero
+        ''').strip() in excinfo.value.args[0]
+
+    vim.command('python3 def raise_error(): raise RuntimeError("oops")')
+    with pytest.raises(NvimError) as excinfo:
+        vim.command_output('python3 =print("nooo", raise_error())')
+    assert textwrap.dedent('''\
+        Traceback (most recent call last):
+          File "<string>", line 1, in <module>
+          File "<string>", line 1, in raise_error
+        RuntimeError: oops
+        ''').strip() in excinfo.value.args[0]
+    assert 'nooo' not in vim.command_output(':messages')
+
+
 def test_python_cwd(vim: Nvim, tmp_path: Path) -> None:
     vim.command('python3 import os')
     cwd_before = vim.command_output('python3 print(os.getcwd())')