Use COMSPEC for shell detection fallback

The subshell logic now considers:

* PIPENV_SHELL
* Auto detection
* SHELL (should very likely work for POSIX)
* PYENV_SHELL
* COMSPEC (should always work for Windows)

pipenv/environments.py

@@ -207,7 +207,11 @@ if "PIPENV_ACTIVE" not in os.environ and not PIPENV_IGNORE_VIRTUALENVS:
 PIPENV_SKIP_VALIDATION = True
 
 # Internal, the default shell to use if shell detection fails.
-PIPENV_SHELL = os.environ.get("SHELL") or os.environ.get("PYENV_SHELL")
+PIPENV_SHELL = (
+    os.environ.get("SHELL") or
+    os.environ.get("PYENV_SHELL") or
+    os.environ.get("COMSPEC")
+)
 
 # Internal, to tell if pyenv is installed.
 PYENV_ROOT = os.environ.get("PYENV_ROOT", os.path.expanduser("~/.pyenv"))

pipenv/shells.py

@@ -29,7 +29,7 @@ def detect_info():
     raise ShellDetectionFailure
 
 
-def _get_activate_script(venv):
+def _get_activate_script(cmd, venv):
     """Returns the string to activate a virtualenv.
 
     This is POSIX-only at the moment since the compat (pexpect-based) shell
@@ -37,11 +37,11 @@ def _get_activate_script(venv):
     """
     # Suffix and source command for other shells.
     # Support for fish shell.
-    if PIPENV_SHELL and "fish" in PIPENV_SHELL:
+    if "fish" in cmd:
         suffix = ".fish"
         command = "source"
     # Support for csh shell.
-    elif PIPENV_SHELL and "csh" in PIPENV_SHELL:
+    elif "csh" in cmd:
         suffix = ".csh"
         command = "source"
     else:
@@ -104,7 +104,7 @@ class Shell(object):
         dims = get_terminal_size()
         with temp_environ():
             c = pexpect.spawn(self.cmd, ["-i"], dimensions=(dims.lines, dims.columns))
-        c.sendline(_get_activate_script(venv))
+        c.sendline(_get_activate_script(self.cmd, venv))
         if args:
             c.sendline(" ".join(args))