fix(cmd): throw GitCommandNotFoundError ...

... if it is not found. Previously, especially on windows, this wasn't
explicit.

Fixes #248, affects #126

doc/source/changes.rst

@@ -12,6 +12,11 @@ Changelog
   these operations to never raise. However, that behavious is undesirable as it would effectively hide the fact that there 
   was an error. See `this issue <https://github.com/gitpython-developers/GitPython/issues/271>`_ for more information.
 
+* If the git executable can't be found in the PATH or at the path provided by `GIT_PYTHON_GIT_EXECUTABLE`, this is made
+  obvious by throwing `GitCommandNotFound`, both on unix and on windows.
+
+  - Those who support **GUI on windows** will now have to set `git.Git.USE_SHELL = True` to get the previous behaviour.
+
 0.3.6 - Features
 ================
 * **DOCS**

git/cmd.py

@@ -26,7 +26,10 @@
     stream_copy,
     WaitGroup
 )
-from .exc import GitCommandError
+from .exc import (
+    GitCommandError,
+    GitCommandNotFound
+)
 from git.compat import (
     string_types,
     defenc,
@@ -241,6 +244,12 @@ class Git(LazyMixin):
     _git_exec_env_var = "GIT_PYTHON_GIT_EXECUTABLE"
     GIT_PYTHON_GIT_EXECUTABLE = os.environ.get(_git_exec_env_var, git_exec_name)
 
+    # If True, a shell will be used when executing git commands.
+    # This should only be desirable on windows, see https://github.com/gitpython-developers/GitPython/pull/126
+    # for more information
+    # Override this value using `Git.USE_SHELL = True`
+    USE_SHELL = False
+
     class AutoInterrupt(object):
 
         """Kill/Interrupt the stored process instance once this instance goes out of scope. It is
@@ -543,18 +552,29 @@ def execute(self, command,
         env["LC_MESSAGES"] = "C"
         env.update(self._environment)
 
-        proc = Popen(command,
-                     env=env,
-                     cwd=cwd,
-                     stdin=istream,
-                     stderr=PIPE,
-                     stdout=PIPE,
-                     # Prevent cmd prompt popups on windows by using a shell ... .
-                     # See https://github.com/gitpython-developers/GitPython/pull/126
-                     shell=sys.platform == 'win32',
-                     close_fds=(os.name == 'posix'),  # unsupported on windows
-                     **subprocess_kwargs
-                     )
+        if sys.platform == 'win32':
+            cmd_not_found_exception = WindowsError
+        else:
+            if sys.version_info[0] > 2:
+                cmd_not_found_exception = FileNotFoundError  # NOQA # this is defined, but flake8 doesn't know
+            else:
+                cmd_not_found_exception = OSError
+        # end handle
+
+        try:
+            proc = Popen(command,
+                         env=env,
+                         cwd=cwd,
+                         stdin=istream,
+                         stderr=PIPE,
+                         stdout=PIPE,
+                         shell=self.USE_SHELL,
+                         close_fds=(os.name == 'posix'),  # unsupported on windows
+                         **subprocess_kwargs
+                         )
+        except cmd_not_found_exception as err:
+            raise GitCommandNotFound(str(err))
+
         if as_process:
             return self.AutoInterrupt(proc, command)
 

git/exc.py

@@ -18,6 +18,12 @@ class NoSuchPathError(OSError):
     """ Thrown if a path could not be access by the system. """
 
 
+class GitCommandNotFound(Exception):
+    """Thrown if we cannot find the `git` executable in the PATH or at the path given by
+    the GIT_PYTHON_GIT_EXECUTABLE environment variable"""
+    pass
+
+
 class GitCommandError(Exception):
     """ Thrown if execution of the git command fails with non-zero status code. """