baenv
Manage ballistica execution environment.
This module is used to set up and/or check the global Python environment before running a ballistica app. This includes things such as paths, logging, and app-dirs. Because these things are global in nature, this should be done before any ballistica modules are imported.
This module can also be exec'ed directly to set up a default environment and then run the app.
Ballistica can be used without explicitly configuring the environment in order to integrate it in arbitrary Python environments, but this may cause some features to be disabled or behave differently than expected.
1# Released under the MIT License. See LICENSE for details. 2# 3"""Manage ballistica execution environment. 4 5This module is used to set up and/or check the global Python environment 6before running a ballistica app. This includes things such as paths, 7logging, and app-dirs. Because these things are global in nature, this 8should be done before any ballistica modules are imported. 9 10This module can also be exec'ed directly to set up a default environment 11and then run the app. 12 13Ballistica can be used without explicitly configuring the environment in 14order to integrate it in arbitrary Python environments, but this may 15cause some features to be disabled or behave differently than expected. 16""" 17from __future__ import annotations 18 19import os 20import sys 21import logging 22from pathlib import Path 23from dataclasses import dataclass 24from typing import TYPE_CHECKING 25import __main__ 26 27if TYPE_CHECKING: 28 from typing import Any 29 30 from efro.log import LogHandler 31 32# IMPORTANT - It is likely (and in some cases expected) that this 33# module's code will be exec'ed multiple times. This is because it is 34# the job of this module to set up Python paths for an engine run, and 35# that may involve modifying sys.path in such a way that this module 36# resolves to a different path afterwards (for example from 37# /abs/path/to/ba_data/scripts/babase.py to ba_data/scripts/babase.py). 38# This can result in the next import of baenv loading us from our 'new' 39# location, which may or may not actually be the same file on disk as 40# the last load. Either way, however, multiple execs will happen in some 41# form. 42# 43# To handle that situation gracefully, we need to do a few things: 44# 45# - First, we need to store any mutable global state in the __main__ 46# module; not in ourself. This way, alternate versions of ourself will 47# still know if we already ran configure/etc. 48# 49# - Second, we should avoid the use of isinstance and similar calls for 50# our types. An EnvConfig we create would technically be a different 51# type than an EnvConfig created by an alternate baenv. 52 53# Build number and version of the ballistica binary we expect to be 54# using. 55TARGET_BALLISTICA_BUILD = 21827 56TARGET_BALLISTICA_VERSION = '1.7.35' 57 58 59@dataclass 60class EnvConfig: 61 """Final config values we provide to the engine.""" 62 63 # Where app config/state data lives. 64 config_dir: str 65 66 # Directory containing ba_data and any other platform-specific data. 67 data_dir: str 68 69 # Where the app's built-in Python stuff lives. 70 app_python_dir: str | None 71 72 # Where the app's built-in Python stuff lives in the default case. 73 standard_app_python_dir: str 74 75 # Where the app's bundled third party Python stuff lives. 76 site_python_dir: str | None 77 78 # Custom Python provided by the user (mods). 79 user_python_dir: str | None 80 81 # We have a mechanism allowing app scripts to be overridden by 82 # placing a specially named directory in a user-scripts dir. 83 # This is true if that is enabled. 84 is_user_app_python_dir: bool 85 86 # Our fancy app log handler. This handles feeding logs, stdout, and 87 # stderr into the engine so they show up on in-app consoles, etc. 88 log_handler: LogHandler | None 89 90 # Initial data from the ballisticakit-config.json file. This is 91 # passed mostly as an optimization to avoid reading the same config 92 # file twice, since config data is first needed in baenv and next in 93 # the engine. It will be cleared after passing it to the app's 94 # config management subsystem and should not be accessed by any 95 # other code. 96 initial_app_config: Any 97 98 99@dataclass 100class _EnvGlobals: 101 """Globals related to baenv's operation. 102 103 We store this in __main__ instead of in our own module because it 104 is likely that multiple versions of our module will be spun up 105 and we want a single set of globals (see notes at top of our module 106 code). 107 """ 108 109 config: EnvConfig | None = None 110 called_configure: bool = False 111 paths_set_failed: bool = False 112 modular_main_called: bool = False 113 114 @classmethod 115 def get(cls) -> _EnvGlobals: 116 """Create/return our singleton.""" 117 name = '_baenv_globals' 118 envglobals: _EnvGlobals | None = getattr(__main__, name, None) 119 if envglobals is None: 120 envglobals = _EnvGlobals() 121 setattr(__main__, name, envglobals) 122 return envglobals 123 124 125def did_paths_set_fail() -> bool: 126 """Did we try to set paths and fail?""" 127 return _EnvGlobals.get().paths_set_failed 128 129 130def config_exists() -> bool: 131 """Has a config been created?""" 132 133 return _EnvGlobals.get().config is not None 134 135 136def get_config() -> EnvConfig: 137 """Return the active config, creating a default if none exists.""" 138 envglobals = _EnvGlobals.get() 139 140 # If configure() has not been explicitly called, set up a 141 # minimally-intrusive default config. We want Ballistica to default 142 # to being a good citizen when imported into alien environments and 143 # not blow away logging or otherwise muck with stuff. All official 144 # paths to run Ballistica apps should be explicitly calling 145 # configure() first to get a full featured setup. 146 if not envglobals.called_configure: 147 configure(setup_logging=False) 148 149 config = envglobals.config 150 if config is None: 151 raise RuntimeError( 152 'baenv.configure() has been called but no config exists;' 153 ' perhaps it errored?' 154 ) 155 return config 156 157 158def configure( 159 config_dir: str | None = None, 160 data_dir: str | None = None, 161 user_python_dir: str | None = None, 162 app_python_dir: str | None = None, 163 site_python_dir: str | None = None, 164 contains_python_dist: bool = False, 165 setup_logging: bool = True, 166) -> None: 167 """Set up the environment for running a Ballistica app. 168 169 This includes things such as Python path wrangling and app directory 170 creation. This must be called before any actual Ballistica modules 171 are imported; the environment is locked in as soon as that happens. 172 """ 173 174 envglobals = _EnvGlobals.get() 175 176 # Keep track of whether we've been *called*, not whether a config 177 # has been created. Otherwise its possible to get multiple 178 # overlapping configure calls going. 179 if envglobals.called_configure: 180 raise RuntimeError( 181 'baenv.configure() has already been called;' 182 ' it can only be called once.' 183 ) 184 envglobals.called_configure = True 185 186 # The very first thing we do is setup Python paths (while also 187 # calculating some engine paths). This code needs to be bulletproof 188 # since we have no logging yet at this point. We used to set up 189 # logging first, but this way logging stuff will get loaded from its 190 # proper final path (otherwise we might wind up using two different 191 # versions of efro.logging in a single engine run). 192 ( 193 user_python_dir, 194 app_python_dir, 195 site_python_dir, 196 data_dir, 197 config_dir, 198 standard_app_python_dir, 199 is_user_app_python_dir, 200 ) = _setup_paths( 201 user_python_dir, 202 app_python_dir, 203 site_python_dir, 204 data_dir, 205 config_dir, 206 ) 207 208 # The second thing we do is set up our logging system and pipe 209 # Python's stdout/stderr into it. At this point we can at least 210 # debug problems on systems where native stdout/stderr is not easily 211 # accessible such as Android. 212 log_handler = _setup_logging() if setup_logging else None 213 214 # We want to always be run in UTF-8 mode; complain if we're not. 215 if sys.flags.utf8_mode != 1: 216 logging.warning( 217 "Python's UTF-8 mode is not set. Running Ballistica without" 218 ' it may lead to errors.' 219 ) 220 221 # Attempt to create dirs that we'll write stuff to. 222 _setup_dirs(config_dir, user_python_dir) 223 224 # Get ssl working if needed so we can use https and all that. 225 _setup_certs(contains_python_dist) 226 227 # This is now the active config. 228 envglobals.config = EnvConfig( 229 config_dir=config_dir, 230 data_dir=data_dir, 231 user_python_dir=user_python_dir, 232 app_python_dir=app_python_dir, 233 standard_app_python_dir=standard_app_python_dir, 234 site_python_dir=site_python_dir, 235 log_handler=log_handler, 236 is_user_app_python_dir=is_user_app_python_dir, 237 initial_app_config=None, 238 ) 239 240 241def _calc_data_dir(data_dir: str | None) -> str: 242 if data_dir is None: 243 # To calc default data_dir, we assume this module was imported 244 # from that dir's ba_data/python subdir. 245 assert Path(__file__).parts[-3:-1] == ('ba_data', 'python') 246 data_dir_path = Path(__file__).parents[2] 247 248 # Prefer tidy relative paths like './ba_data' if possible so 249 # that things like stack traces are easier to read. For best 250 # results, platforms where CWD doesn't matter can chdir to where 251 # ba_data lives before calling configure(). 252 # 253 # NOTE: If there's ever a case where the user is chdir'ing at 254 # runtime we might want an option to use only abs paths here. 255 cwd_path = Path.cwd() 256 data_dir = str( 257 data_dir_path.relative_to(cwd_path) 258 if data_dir_path.is_relative_to(cwd_path) 259 else data_dir_path 260 ) 261 return data_dir 262 263 264def _setup_logging() -> LogHandler: 265 from efro.log import setup_logging, LogLevel 266 267 # TODO: should set this up with individual loggers under a top level 268 # 'ba' logger, and at that point we can kill off the 269 # suppress_non_root_debug option since we'll only ever need to set 270 # 'ba' to DEBUG at most. 271 log_handler = setup_logging( 272 log_path=None, 273 level=LogLevel.DEBUG, 274 suppress_non_root_debug=True, 275 log_stdout_stderr=True, 276 cache_size_limit=1024 * 1024, 277 ) 278 return log_handler 279 280 281def _setup_certs(contains_python_dist: bool) -> None: 282 # In situations where we're bringing our own Python, let's also 283 # provide our own root certs so ssl works. We can consider 284 # overriding this in particular embedded cases if we can verify that 285 # system certs are working. We also allow forcing this via an env 286 # var if the user desires. 287 if ( 288 contains_python_dist 289 or os.environ.get('BA_USE_BUNDLED_ROOT_CERTS') == '1' 290 ): 291 import certifi 292 293 # Let both OpenSSL and requests (if present) know to use this. 294 os.environ['SSL_CERT_FILE'] = os.environ['REQUESTS_CA_BUNDLE'] = ( 295 certifi.where() 296 ) 297 298 299def _setup_paths( 300 user_python_dir: str | None, 301 app_python_dir: str | None, 302 site_python_dir: str | None, 303 data_dir: str | None, 304 config_dir: str | None, 305) -> tuple[str | None, str | None, str | None, str, str, str, bool]: 306 # First a few paths we can ALWAYS calculate since they don't affect 307 # Python imports: 308 309 envglobals = _EnvGlobals.get() 310 311 data_dir = _calc_data_dir(data_dir) 312 313 # Default config-dir is simply ~/.ballisticakit 314 if config_dir is None: 315 config_dir = str(Path(Path.home(), '.ballisticakit')) 316 317 # Standard app-python-dir is simply ba_data/python under data-dir. 318 standard_app_python_dir = str(Path(data_dir, 'ba_data', 'python')) 319 320 # Whether the final app-dir we're returning is a custom user-owned one. 321 is_user_app_python_dir = False 322 323 # If _babase has already been imported, there's not much we can do 324 # at this point aside from complain and inform for next time. 325 if '_babase' in sys.modules: 326 app_python_dir = user_python_dir = site_python_dir = None 327 328 # We don't actually complain yet here; we simply take note that 329 # we weren't able to set paths. Then we complain if/when the app 330 # is started. This way, non-app uses of babase won't be filled 331 # with unnecessary warnings. 332 envglobals.paths_set_failed = True 333 334 else: 335 # Ok; _babase hasn't been imported yet, so we can muck with 336 # Python paths. 337 338 if app_python_dir is None: 339 app_python_dir = standard_app_python_dir 340 341 # Likewise site-python-dir defaults to ba_data/python-site-packages. 342 if site_python_dir is None: 343 site_python_dir = str( 344 Path(data_dir, 'ba_data', 'python-site-packages') 345 ) 346 347 # By default, user-python-dir is simply 'mods' under config-dir. 348 if user_python_dir is None: 349 user_python_dir = str(Path(config_dir, 'mods')) 350 351 # Wherever our user_python_dir is, if we find a sys/FOO dir 352 # under it where FOO matches our version, use that as our 353 # app_python_dir. This allows modding built-in stuff on 354 # platforms where there is no write access to said built-in 355 # stuff. 356 check_dir = Path(user_python_dir, 'sys', TARGET_BALLISTICA_VERSION) 357 try: 358 if check_dir.is_dir(): 359 app_python_dir = str(check_dir) 360 is_user_app_python_dir = True 361 except PermissionError: 362 logging.warning( 363 "PermissionError checking user-app-python-dir path '%s'.", 364 check_dir, 365 ) 366 367 # Ok, now apply these to sys.path. 368 369 # First off, strip out any instances of the path containing this 370 # module. We will *probably* be re-adding the same path in a 371 # moment so this keeps things cleaner. Though hmm should we 372 # leave it in there in cases where we *don't* re-add the same 373 # path?... 374 our_parent_path = Path(__file__).parent.resolve() 375 oldpaths: list[str] = [ 376 p for p in sys.path if Path(p).resolve() != our_parent_path 377 ] 378 379 # Let's place mods first (so users can override whatever they 380 # want) followed by our app scripts and lastly our bundled site 381 # stuff. 382 383 # One could make the argument that at least our bundled app & 384 # site stuff should be placed at the end so actual local site 385 # stuff could override it. That could be a good thing or a bad 386 # thing. Maybe we could add an option for that, but for now I'm 387 # prioritizing our stuff to give as consistent an environment as 388 # possible. 389 ourpaths = [user_python_dir, app_python_dir, site_python_dir] 390 391 # Special case: our modular builds will have a 'python-dylib' 392 # dir alongside the 'python' scripts dir which contains our 393 # binary Python modules. If we see that, add it to the path also. 394 # Not sure if we'd ever have a need to customize this path. 395 dylibdir = f'{app_python_dir}-dylib' 396 if os.path.exists(dylibdir): 397 ourpaths.append(dylibdir) 398 399 sys.path = ourpaths + oldpaths 400 401 return ( 402 user_python_dir, 403 app_python_dir, 404 site_python_dir, 405 data_dir, 406 config_dir, 407 standard_app_python_dir, 408 is_user_app_python_dir, 409 ) 410 411 412def _setup_dirs(config_dir: str | None, user_python_dir: str | None) -> None: 413 create_dirs: list[tuple[str, str | None]] = [ 414 ('config', config_dir), 415 ('user_python', user_python_dir), 416 ] 417 for cdirname, cdir in create_dirs: 418 if cdir is not None: 419 try: 420 os.makedirs(cdir, exist_ok=True) 421 except Exception: 422 # Not the end of the world if we can't make these dirs. 423 logging.warning( 424 "Unable to create %s dir at '%s'.", cdirname, cdir 425 ) 426 427 428def extract_arg(args: list[str], names: list[str], is_dir: bool) -> str | None: 429 """Given a list of args and an arg name, returns a value. 430 431 The arg flag and value are removed from the arg list. We also check 432 to make sure the path exists. 433 434 raises CleanErrors on any problems. 435 """ 436 from efro.error import CleanError 437 438 count = sum(args.count(n) for n in names) 439 if not count: 440 return None 441 442 if count > 1: 443 raise CleanError(f'Arg {names} passed multiple times.') 444 445 for name in names: 446 if name not in args: 447 continue 448 argindex = args.index(name) 449 if argindex + 1 >= len(args): 450 raise CleanError(f'No value passed after {name} arg.') 451 452 val = args[argindex + 1] 453 del args[argindex : argindex + 2] 454 455 if is_dir and not os.path.isdir(val): 456 namepretty = names[0].removeprefix('--') 457 raise CleanError( 458 f"Provided {namepretty} path '{val}' is not a directory." 459 ) 460 return val 461 462 raise RuntimeError(f'Expected arg name not found from {names}') 463 464 465def _modular_main() -> None: 466 from efro.error import CleanError 467 468 # Fundamentally, running a Ballistica app consists of the following: 469 # import baenv; baenv.configure(); import babase; babase.app.run() 470 # 471 # First baenv sets up things like Python paths the way the engine 472 # needs them, and then we import and run the engine. 473 # 474 # Below we're doing a slightly fancier version of that. Namely, we 475 # do some processing of command line args to allow overriding of 476 # paths or running explicit commands or whatever else. Our goal is 477 # that this modular form of the app should be basically 478 # indistinguishable from the monolithic form when used from the 479 # command line. 480 481 try: 482 # Take note that we're running via modular-main. The native 483 # layer can key off this to know whether it should apply 484 # sys.argv or not. 485 _EnvGlobals.get().modular_main_called = True 486 487 # Deal with a few key things here ourself before even running 488 # configure. 489 490 # The extract_arg stuff below modifies this so we work with a 491 # copy. 492 args = sys.argv.copy() 493 494 # NOTE: We need to keep these arg long/short arg versions synced 495 # to those in core_config.cc. That code parses these same args 496 # (even if it doesn't handle them in our case) and will complain 497 # if unrecognized args come through. 498 499 # Our -c arg basically mirrors Python's -c arg. If we get that, 500 # simply exec it and return; no engine stuff. 501 command = extract_arg(args, ['--command', '-c'], is_dir=False) 502 if command is not None: 503 exec(command) # pylint: disable=exec-used 504 return 505 506 config_dir = extract_arg(args, ['--config-dir', '-C'], is_dir=True) 507 data_dir = extract_arg(args, ['--data-dir', '-d'], is_dir=True) 508 mods_dir = extract_arg(args, ['--mods-dir', '-m'], is_dir=True) 509 510 # We run configure() BEFORE importing babase. (part of its job 511 # is to wrangle paths which can affect where babase and 512 # everything else gets loaded from). 513 configure( 514 config_dir=config_dir, 515 data_dir=data_dir, 516 user_python_dir=mods_dir, 517 ) 518 519 import babase 520 521 # The engine will have parsed and processed all other args as 522 # part of the above import. If there were errors or args such as 523 # --help which should lead to us immediately returning, do so. 524 code = babase.get_immediate_return_code() 525 if code is not None: 526 sys.exit(code) 527 528 # Aaaand we're off! 529 babase.app.run() 530 531 # Code wanting us to die with a clean error message instead of an 532 # ugly stack trace can raise one of these. 533 except CleanError as clean_exc: 534 clean_exc.pretty_print() 535 sys.exit(1) 536 537 538# Exec'ing this module directly will do a standard app run. 539if __name__ == '__main__': 540 _modular_main()
60@dataclass 61class EnvConfig: 62 """Final config values we provide to the engine.""" 63 64 # Where app config/state data lives. 65 config_dir: str 66 67 # Directory containing ba_data and any other platform-specific data. 68 data_dir: str 69 70 # Where the app's built-in Python stuff lives. 71 app_python_dir: str | None 72 73 # Where the app's built-in Python stuff lives in the default case. 74 standard_app_python_dir: str 75 76 # Where the app's bundled third party Python stuff lives. 77 site_python_dir: str | None 78 79 # Custom Python provided by the user (mods). 80 user_python_dir: str | None 81 82 # We have a mechanism allowing app scripts to be overridden by 83 # placing a specially named directory in a user-scripts dir. 84 # This is true if that is enabled. 85 is_user_app_python_dir: bool 86 87 # Our fancy app log handler. This handles feeding logs, stdout, and 88 # stderr into the engine so they show up on in-app consoles, etc. 89 log_handler: LogHandler | None 90 91 # Initial data from the ballisticakit-config.json file. This is 92 # passed mostly as an optimization to avoid reading the same config 93 # file twice, since config data is first needed in baenv and next in 94 # the engine. It will be cleared after passing it to the app's 95 # config management subsystem and should not be accessed by any 96 # other code. 97 initial_app_config: Any
Final config values we provide to the engine.
126def did_paths_set_fail() -> bool: 127 """Did we try to set paths and fail?""" 128 return _EnvGlobals.get().paths_set_failed
Did we try to set paths and fail?
131def config_exists() -> bool: 132 """Has a config been created?""" 133 134 return _EnvGlobals.get().config is not None
Has a config been created?
137def get_config() -> EnvConfig: 138 """Return the active config, creating a default if none exists.""" 139 envglobals = _EnvGlobals.get() 140 141 # If configure() has not been explicitly called, set up a 142 # minimally-intrusive default config. We want Ballistica to default 143 # to being a good citizen when imported into alien environments and 144 # not blow away logging or otherwise muck with stuff. All official 145 # paths to run Ballistica apps should be explicitly calling 146 # configure() first to get a full featured setup. 147 if not envglobals.called_configure: 148 configure(setup_logging=False) 149 150 config = envglobals.config 151 if config is None: 152 raise RuntimeError( 153 'baenv.configure() has been called but no config exists;' 154 ' perhaps it errored?' 155 ) 156 return config
Return the active config, creating a default if none exists.
159def configure( 160 config_dir: str | None = None, 161 data_dir: str | None = None, 162 user_python_dir: str | None = None, 163 app_python_dir: str | None = None, 164 site_python_dir: str | None = None, 165 contains_python_dist: bool = False, 166 setup_logging: bool = True, 167) -> None: 168 """Set up the environment for running a Ballistica app. 169 170 This includes things such as Python path wrangling and app directory 171 creation. This must be called before any actual Ballistica modules 172 are imported; the environment is locked in as soon as that happens. 173 """ 174 175 envglobals = _EnvGlobals.get() 176 177 # Keep track of whether we've been *called*, not whether a config 178 # has been created. Otherwise its possible to get multiple 179 # overlapping configure calls going. 180 if envglobals.called_configure: 181 raise RuntimeError( 182 'baenv.configure() has already been called;' 183 ' it can only be called once.' 184 ) 185 envglobals.called_configure = True 186 187 # The very first thing we do is setup Python paths (while also 188 # calculating some engine paths). This code needs to be bulletproof 189 # since we have no logging yet at this point. We used to set up 190 # logging first, but this way logging stuff will get loaded from its 191 # proper final path (otherwise we might wind up using two different 192 # versions of efro.logging in a single engine run). 193 ( 194 user_python_dir, 195 app_python_dir, 196 site_python_dir, 197 data_dir, 198 config_dir, 199 standard_app_python_dir, 200 is_user_app_python_dir, 201 ) = _setup_paths( 202 user_python_dir, 203 app_python_dir, 204 site_python_dir, 205 data_dir, 206 config_dir, 207 ) 208 209 # The second thing we do is set up our logging system and pipe 210 # Python's stdout/stderr into it. At this point we can at least 211 # debug problems on systems where native stdout/stderr is not easily 212 # accessible such as Android. 213 log_handler = _setup_logging() if setup_logging else None 214 215 # We want to always be run in UTF-8 mode; complain if we're not. 216 if sys.flags.utf8_mode != 1: 217 logging.warning( 218 "Python's UTF-8 mode is not set. Running Ballistica without" 219 ' it may lead to errors.' 220 ) 221 222 # Attempt to create dirs that we'll write stuff to. 223 _setup_dirs(config_dir, user_python_dir) 224 225 # Get ssl working if needed so we can use https and all that. 226 _setup_certs(contains_python_dist) 227 228 # This is now the active config. 229 envglobals.config = EnvConfig( 230 config_dir=config_dir, 231 data_dir=data_dir, 232 user_python_dir=user_python_dir, 233 app_python_dir=app_python_dir, 234 standard_app_python_dir=standard_app_python_dir, 235 site_python_dir=site_python_dir, 236 log_handler=log_handler, 237 is_user_app_python_dir=is_user_app_python_dir, 238 initial_app_config=None, 239 )
Set up the environment for running a Ballistica app.
This includes things such as Python path wrangling and app directory creation. This must be called before any actual Ballistica modules are imported; the environment is locked in as soon as that happens.
429def extract_arg(args: list[str], names: list[str], is_dir: bool) -> str | None: 430 """Given a list of args and an arg name, returns a value. 431 432 The arg flag and value are removed from the arg list. We also check 433 to make sure the path exists. 434 435 raises CleanErrors on any problems. 436 """ 437 from efro.error import CleanError 438 439 count = sum(args.count(n) for n in names) 440 if not count: 441 return None 442 443 if count > 1: 444 raise CleanError(f'Arg {names} passed multiple times.') 445 446 for name in names: 447 if name not in args: 448 continue 449 argindex = args.index(name) 450 if argindex + 1 >= len(args): 451 raise CleanError(f'No value passed after {name} arg.') 452 453 val = args[argindex + 1] 454 del args[argindex : argindex + 2] 455 456 if is_dir and not os.path.isdir(val): 457 namepretty = names[0].removeprefix('--') 458 raise CleanError( 459 f"Provided {namepretty} path '{val}' is not a directory." 460 ) 461 return val 462 463 raise RuntimeError(f'Expected arg name not found from {names}')
Given a list of args and an arg name, returns a value.
The arg flag and value are removed from the arg list. We also check to make sure the path exists.
raises CleanErrors on any problems.