Prefixed Method Does Not Clear self._prefix On Error

[rcuza]

version: 4.1.3

Problem

When using the following code, the private variable self._prefix is not cleared on errors:

with env.prefixed(PREFIX):
    do_something()

If do_something() raises an error, then session attempts to run with env.prefixed() again it will behave like it is a nested with. This can happen in an interactive shell or if exceptions are caught.

Details

You can do the following to reproduce.

$ export PREFIX_PNG="path/to/my.png"
$ ipython
/usr/local/Cellar/ipython/7.5.0/libexec/lib/python3.7/site-packages/IPython/core/interactiveshell.py:925: UserWarning: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
  warn("Attempting to work in a virtualenv. If you encounter problems, please "
Python 3.7.3 (default, Mar 27 2019, 09:23:15)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.5.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from environs import Env

In [2]: env = Env()

In [3]: with env.prefixed("PREFIX_"):
   ...:     print(env.str("PNG"))
   ...:
path/to/my.png

In [4]: with env.prefixed("PREFIX_"):
   ...:     raise
   ...:
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-4-6c7273e50e26> in <module>
      1 with env.prefixed("PREFIX_"):
----> 2     raise
      3

RuntimeError: No active exception to reraise

In [5]: with env.prefixed("PREFIX_"):
   ...:     print(env.str("PNG"))
   ...:
---------------------------------------------------------------------------
EnvError                                  Traceback (most recent call last)
<ipython-input-5-60d974fcda59> in <module>
      1 with env.prefixed("PREFIX_"):
----> 2     print(env.str("PNG"))
      3

~/dev/virtualenvs/tmp-da5b0d36b01c294/lib/python3.7/site-packages/environs.py in method(self, name, default, subcast, **kwargs)
     51         if raw_value is ma.missing and field.missing is ma.missing:
     52             raise EnvError(
---> 53                 'Environment variable "{}" not set'.format(proxied_key or parsed_key)
     54             )
     55         if raw_value or raw_value == "":

EnvError: Environment variable "PREFIX_PREFIX_PNG" not set

[rcuza]

The workaround for this issue is to set env._prefix = None if you are using env = Env().