argparse - specify order of argument parsing?

[Eduardo Alvarez]

When using argparse, is there a way to specify in what order arguments
get parsed? I am writing a script whose parameters can be modified in
the following order:

Defaults -> config file -> command-line switches.

However, I want to give the option of specifying a config file using a
command line switch as well, so logically, that file should be parsed
before any other arguments are applied. However, it seems that
parse_args() parses arguments in the order they're given, so if the
config file switch is not given first, the config file will overwrite
whatever was in the command-line switches, which should have higher
priority.

Thank you in advance,

 

[Tim Chase]

When using argparse, is there a way to specify in what order
arguments get parsed? I am writing a script whose parameters can be
modified in the following order:

Defaults -> config file -> command-line switches.

However, I want to give the option of specifying a config file
using a command line switch as well, so logically, that file should
be parsed before any other arguments are applied. However, it seems
that parse_args() parses arguments in the order they're given, so
if the config file switch is not given first, the config file will
overwrite whatever was in the command-line switches, which should
have higher priority.

While I haven't come up with a good solution using argparse/optparse
alone, I've found that it's easier (for processing) to specify the
config file as an environment variable. So rather than doing

my_prog.py -c /path/to/config.ini arg1 arg2 ...

I just do

MY_PROG_CONF=/path/to/config.ini my_prog.py arg1 arg2 ...

....at least on *nix systems; on Win32, it's

c:\temp> set MY_PROG_CONF=c:\path\to\config.ini
c:\temp> python my_prog.py arg1 arg2 ...

Then you just intercept the config-file name from os.environ:

config_file = os.environ.get(
"MY_PROG_CONF",
default_ini_location)

-tkc

 

[Terry Reedy]

When using argparse, is there a way to specify in what order arguments
get parsed?

I expect argparse to forward iterate the sequence of arguments that it
receives.

I am writing a script whose parameters can be modified in
the following order:

Defaults -> config file -> command-line switches.

However, I want to give the option of specifying a config file using a
command line switch as well, so logically, that file should be parsed
before any other arguments are applied. However, it seems that
parse_args() parses arguments in the order they're given,
Right.

so if the
config file switch is not given first, the config file will overwrite
whatever was in the command-line switches, which should have higher
priority.

So just document that a config file has to be given first to work as
expected. Order dependence among arguments is common.

 

[Terry Reedy]

I expect argparse to forward iterate the sequence of arguments that it
receives.

Aside from the environment variable solution, you could search sys.argv
for 'config=filename' and remove it and process it *before* you invoke
argparse.

 

[Peter Otten]

When using argparse, is there a way to specify in what order arguments
get parsed? I am writing a script whose parameters can be modified in
the following order:

Defaults -> config file -> command-line switches.

However, I want to give the option of specifying a config file using a
command line switch as well, so logically, that file should be parsed
before any other arguments are applied. However, it seems that
parse_args() parses arguments in the order they're given, so if the
config file switch is not given first, the config file will overwrite
whatever was in the command-line switches, which should have higher
priority.

Thank you in advance,

If you use

http://docs.python.org/dev/library/argparse.html#fromfile-prefix-chars

to read the configuration file it should be obvious to the user that the
order is significant. You can even construct multiple config files with
partially overlapping options:

$ cat load_options.py
import argparse

parser = argparse.ArgumentParser(fromfile_prefix_chars="@")
parser.add_argument("--infile")
parser.add_argument("--outfile")
parser.add_argument("--logfile")

print(parser.parse_args())
$ cat option1.txt
--infile=alpha.txt
--outfile=beta.txt
$ cat option2.txt
--outfile=GAMMA.txt
--logfile=DELTA.txt
$ python load_options.py @option1.txt @option2.txt
Namespace(infile='alpha.txt', logfile='DELTA.txt', outfile='GAMMA.txt')
$ python load_options.py @option2.txt @option1.txt
Namespace(infile='alpha.txt', logfile='DELTA.txt', outfile='beta.txt')

If you insist you could modify the argument list with the following hack:

sys.argv[1:] = sorted(sys.argv[1:], key=lambda arg: arg.startswith("@"),
reverse=True)

There might also be a way to utilize parse_known_args().