Step 5: Additional features¶

Finally, we include two additional parameters in our calculator.

The user can choose between addition and multiplication. The default operation is addition ("+").
The user can select the number of displayed digits, the default value is 3.

Note the following new features.

Parameter from a list of choices

One can construct a parser of strings using the from_choices() static method of Param. The possible choices are displayed in the usage description when invoking the script help.
Default values

Default values are given as strings, and they are parsed by configpile using the same parser used for other strings. Though if there is a problem, we do not report an error but rather throw an exception.
Environment variables

Parameters can be set from environment variables, though this is disabled by default.

Set the configpile.arg.Param.env_var_name field to enable this feature, and see how to use it in the sample executions below. Note that one can export DIGITS=10 in Bash scripts as well.
Validation

By applying the validated() method on a parser, one can ensure that the final value in the configuration obeys constraints.

Code¶

"""
Calculator tutorial, step 5
"""
from dataclasses import dataclass
from math import prod
from typing import Sequence

from typing_extensions import Annotated

from configpile import Config, Param, Positional, parsers


@dataclass(frozen=True)
class Calc(Config):
    """
    Command-line tool that sums an arbitrary number of floating point values
    """

    #: Values to sum
    values: Annotated[
        Sequence[float],
        Param.append1(
            parsers.float_parser,
            positional=Positional.ZERO_OR_MORE,
            short_flag_name=None,
            long_flag_name=None,
        ),
    ]

    #: Operation to perform, either addition "+" or multiplication "*"
    operation: Annotated[
        str,
        Param.store(
            parsers.Parser.from_choices(["+", "*"]),
            default_value="+",
        ),
    ]

    #: Number of digits to display
    #:
    #: This number of digits can also be set with the environment variable
    #: ``DIGITS``
    digits: Annotated[
        int,
        Param.store(
            parsers.int_parser.validated(lambda i: i > 0, "Must be a positive integer"),
            default_value="3",
            env_var_name="DIGITS",
        ),
    ]


c = Calc.from_command_line_()
fmt_string = f"%.{c.digits}f"
if c.operation == "+":
    print(fmt_string % sum(c.values))
elif c.operation == "*":
    print(fmt_string % prod(c.values))

Executions¶

$ python ../../examples/calculator5.py --help
usage: ../../examples/calculator5.py [--operation {+,*}] [--digits DIGITS]
                                     values

Command-line tool that sums an arbitrary number of floating point values

optional arguments:
  values             Values to sum
  --operation {+,*}  Operation to perform, either addition "+" or multiplication "*"
  --digits DIGITS    Number of digits to display
                     
                     This number of digits can also be set with the environment variable
                     ``DIGITS``

$ python ../../examples/calculator5.py 1 2 3 4 --operation "*"
24.000

$ bash -c "DIGITS=10 python ../../examples/calculator5.py 1 2 3 4"
10.0000000000

configpile

Step 5: Additional features

Contents

Step 5: Additional features¶

Code¶

Executions¶