Here's a relatively simple (looks longer because it's well-commented) yet complete solution that avoids kludging default by using the namespace argument of parse_args. By default it parses environment variables no differently than command-line arguments though that can easily be changed.
import shlex
# Notes:
# * Based on https://github.com/python/cpython/blob/
# 15bde92e47e824369ee71e30b07f1624396f5cdc/
# Lib/argparse.py
# * Haven't looked into handling "required" for mutually exclusive groups
# * Probably should make new attributes private even though it's ugly.
class EnvArgParser(argparse.ArgumentParser):
# env_k: The keyword to "add_argument" as well as the attribute stored
# on matching actions.
# env_f: The keyword to "add_argument". Defaults to "env_var_parse" if
# not provided.
# env_i: Basic container type to identify unfilled arguments.
env_k = "env_var"
env_f = "env_var_parse"
env_i = type("env_i", (object,), {})
def add_argument(self, *args, **kwargs):
map_f = (lambda m,k,f=None,d=False:
(k, k in m, m.pop(k,f) if d else m.get(k,f)))
env_k = map_f(kwargs, self.env_k, d=True, f="")
env_f = map_f(kwargs, self.env_f, d=True, f=self.env_var_parse)
if env_k[1] and not isinstance(env_k[2], str):
raise ValueError(f"Parameter '{env_k[0]}' must be a string.")
if env_f[1] and not env_k[1]:
raise ValueError(f"Parameter '{env_f[0]}' requires '{env_k[0]}'.")
if env_f[1] and not callable(env_f[2]):
raise ValueError(f"Parameter '{env_f[0]}' must be callable.")
action = super().add_argument(*args, **kwargs)
if env_k[1] and not action.option_strings:
raise ValueError(f"Positional parameters may not specify '{env_k[0]}'.")
# We can get the environment now:
# * We need to know now if the keys exist anyway
# * os.environ is static
env_v = map_f(os.environ, env_k[2], f="")
# Examples:
# env_k:
# ("env_var", True, "FOO_KEY")
# env_v:
# ("FOO_KEY", False, "")
# ("FOO_KEY", True, "FOO_VALUE")
#
# env_k:
# ("env_var", False, "")
# env_v:
# ("" , False, "")
# ("", True, "RIDICULOUS_VALUE")
# Add the identifier to all valid environment variable actions for
# later access by i.e. the help formatter.
if env_k[1]:
if env_v[1] and action.required:
action.required = False
i = self.env_i()
i.a = action
i.k = env_k[2]
i.f = env_f[2]
i.v = env_v[2]
i.p = env_v[1]
setattr(action, env_k[0], i)
return action
# Overriding "_parse_known_args" is better than "parse_known_args":
# * The namespace will already have been created.
# * This method runs in an exception handler.
def _parse_known_args(self, arg_strings, namespace):
"""precedence: cmd args > env var > preexisting namespace > defaults"""
for action in self._actions:
if action.dest is argparse.SUPPRESS:
continue
try:
i = getattr(action, self.env_k)
except AttributeError:
continue
if not i.p:
continue
setattr(namespace, action.dest, i)
namespace, arg_extras = super()._parse_known_args(arg_strings, namespace)
for k,v in vars(namespace).copy().items():
# Setting "env_i" on the action is more effective than using an
# empty unique object() and mapping namespace attributes back to
# actions.
if isinstance(v, self.env_i):
fv = v.f(v.a, v.k, v.v, arg_extras)
if fv is argparse.SUPPRESS:
delattr(namespace, k)
else:
# "_parse_known_args::take_action" checks for action
# conflicts. For simplicity we don't.
v.a(self, namespace, fv, v.k)
return (namespace, arg_extras)
def env_var_parse(self, a, k, v, e):
# Use shlex, yaml, whatever.
v = shlex.split(v)
# From "_parse_known_args::consume_optional".
n = self._match_argument(a, "A"*len(v))
# From the main loop of "_parse_known_args". Treat additional
# environment variable arguments just like additional command-line
# arguments (which will eventually raise an exception).
e.extend(v[n:])
return self._get_values(a, v[:n])
# Derived from "ArgumentDefaultsHelpFormatter".
class EnvArgHelpFormatter(argparse.HelpFormatter):
"""Help message formatter which adds environment variable keys to
argument help.
"""
env_k = EnvArgParser.env_k
# This is supposed to return a %-style format string for "_expand_help".
# Since %-style strings don't support attribute access we instead expand
# "env_k" ourselves.
def _get_help_string(self, a):
h = super()._get_help_string(a)
try:
i = getattr(a, self.env_k)
except AttributeError:
return h
s = f" ({self.env_k}: {i.k})"
if s not in h:
h += s
return h
# An example mix-in.
class DefEnvArgHelpFormatter\
( EnvArgHelpFormatter
, argparse.ArgumentDefaultsHelpFormatter
):
pass
Example program:
parser = EnvArgParser\
( prog="Test Program"
, formatter_class=DefEnvArgHelpFormatter
)
parser.add_argument\
( '--bar'
, required=True
, env_var="BAR"
, type=int
, nargs="+"
, default=22
, help="Help message for bar."
)
parser.add_argument\
( 'baz'
, type=int
)
args = parser.parse_args()
print(args)
Example program output:
$ BAR="1 2 3 '45 ' 6 7" ./envargparse.py 123
Namespace(bar=[1, 2, 3, 45, 6, 7], baz=123)
$ ./envargparse.py -h
usage: Test Program [-h] --bar BAR [BAR ...] baz
positional arguments:
baz
optional arguments:
-h, --help show this help message and exit
--bar BAR [BAR ...] Help message for bar. (default: 22) (env_var: BAR)
