CLI Option Guideline

This document describes the conventions of neutron CLI options.

General conventions

1. Option names should be delimited by a hyphen instead of a underscore. This is the common guidelines across all OpenStack CLIs.
   • Good: --ip-version
   • Not Good: --ip_version
2. Use at least one required option for *-create command. If all options are optional, we typically use name field as a required option.
3. When you need to specify an ID of a resource, it is better to provide another way to specify the resource like name or other reasonable field.
4. If an attribute name in API is foo_id, the corresponding option should be --foo instead of --foo-id.
   • It is because we usually support ID and name to specify a resource.
5. Do not use nargs='?' without a special reason.
   • The behavior of nargs='?' option for python argparse is bit tricky and may lead to unexpected option parsing different from the help message. The detail is described in the Background section below.
6. (option) Avoid using positional options as much as possible.
   • Positional arguments should be limited to attributes which will be required in the long future.
7. We honor existing options and should keep compatibilities when adding or changing options.

Options for boolean value

Use the form of --option-name {True|False}.

• For a new option, it is recommended.
• It is suggested to use common.utils.add_boolean_argument in an implementation. It allows true/false in addition to True/False.
• For existing options, migration to the recommended form is not necessarily required. All backward-compatibility should be kept without reasonable reasons.

Options for dict value

Some API attributes take a dictionary.

--foo key1=val1,key2=val2 is usually used.

This means {"key1": "val1", "key2": "val2"} is passed in the API layer.

Examples:

• --host-route destination=CIDR,nexthop=IP_ADDR for a subnet
• --fixed-ip subnet_id=SUBNET,ip_address=IP_ADDR for a port.

Options for list value

Some attributes take a list.

In this case, we usually use:

• Define an option per element (Use a singular form as an option name)
• Allow to specify the option multiple times

For Example, port-create has --security-group option. --security-group SG1 --security-group SG2 generates {"security_groups: ["SG1", "SG2"]} in the API layer.

This convention applies to a case of a list of dict. --allocation-pool and --host-route for a subnet are examples.

Compatibility with extra arguments

extra arguments supports various types of option specifications. At least the following patterns needs to be considered when defining a new option. For more detail, see Extra arguments for create/update operation.

• Normal options with value
• Boolean options : --foo True, --bar=False
• List options : --bars list=true val1 val2, --bars val1 val2
• Dict options : --foo type=dict key1=va1,key2=val2
• List of Dict options : --bars list=true type=dict key1=val1,key2=val2 key3=val3,key4=val4
• action=clear

For normal options with value, there are four patterns to specify an option as extra arguments.

• --admin-state-up True (a space between option name and value)
• --admin-state-up=True (= between option name and value)
• --admin_state_up True (underscore is used as delimiter)
• --admin_state_up=True (underscore is used as delimiter)

Background

There are a lot of opinions on which form of options are better or not. This section tries to capture the reason of the current choice.

Use at least one required option

As a convention, neutron CLI requires one required argument.

If all options are optional in the API level and we have name field, we usually use name as a required parameter. Requiring at least one argument has the following benefits:

• If we run neutron *-create without a required argument, we will have a brief help message without detail option help. It is convenient.
• We can avoid miss operation by just hitting neutron *-create. Requiring at least one parameter is a good balance.

Even though we can change this convention to allow to create a resource without name field, it will bring confusions to existing users.

There may be opinion that it is inconsistent with API level requirement or Horizon behavior, but even if neutron CLI requires name field there is no bad impact on regular users. Considering possible confusion if we change it, it looks better to keep it as-is.

Options for Boolean value

• --enable-foo/--disable-foo or similar patterns (including --admin-state-down) is not suggested because we need two exclusive options for one attribute in REST API. It is meaningless.
• It is not recommended to have an option only to specify non-default value. For example, we have --shared or --admin-state-down options for net-create. This form only works for *-create and does not work for *-update. It leads to having different options for *-create and *-update.
• A flag option like --enable-dhcp (without value) also has a problem when considering the compatibility with extra argument. We can specify -enable-dhcp True/False or --enable-dhcp=True/False in the extra argument mechanism. If we introduce --enable-dhcp (without value), the form of -enable-dhcp True/False cannot be used now. This is another reason we don’t use a flag style option for a boolean parameter.

Avoid using nargs in positional or optional arguments

The behavior of nargs='?' option for python argparse is bit tricky. When we use nargs='?' and if the order of command-line options is changed then the command-line parser may fail to parse the arguments correctly. Two examples of such failures are provided below.

Example 1: This example shows how the actual behavior can differ from the provided help message. In the below block, help message at [5] says --bb CC is a valid format but the argument parsing for the same format fails at [7].

In [1]: import argparse
In [2]: parser = argparse.ArgumentParser()
In [3]: parser.add_argument('--bb', nargs='?')
In [4]: parser.add_argument('cc')

In [5]: parser.print_help()
usage: ipython [-h] [--bb [BB]] cc

positional arguments:
  cc

optional arguments:
  -h, --help  show this help message and exit
  --bb [BB]

In [6]: parser.parse_args('--bb 1 X'.split())
Out[6]: Namespace(bb='1', cc='X')

In [7]: parser.parse_args('--bb X'.split())
usage: ipython [-h] [--bb [BB]] cc
ipython: error: too few arguments
An exception has occurred, use %tb to see the full traceback.

SystemExit: 2

Example 2: This example shows how fragile nargs='?' can be when user specifies options in different order from the help message.

In [1]: import argparse
In [2]: parser = argparse.ArgumentParser()
In [3]: parser.add_argument('--a', help='option a')
In [4]: parser.add_argument('--b', help='option b')
In [5]: parser.add_argument('x', help='positional arg X')
In [6]: parser.add_argument('y', nargs='?', help='positional arg Y')
In [7]: parser.print_help()
usage: ipython [-h] [--a A] [--b B] x [y]

positional arguments:
  x           positional arg X
  y           positional arg Y

optional arguments:
  -h, --help  show this help message and exit
  --a A       option a
  --b B       option b

In [8]: parser.parse_args('--a 1 --b 2 X Y'.split())
Out[8]: Namespace(a='1', b='2', x='X', y='Y')

In [9]: parser.parse_args('X Y --a 1 --b 2'.split())
Out[9]: Namespace(a='1', b='2', x='X', y='Y')

In [10]: parser.parse_args('X --a 1 --b 2 Y'.split())
usage: ipython [-h] [--a A] [--b B] x [y]
ipython: error: unrecognized arguments: Y
An exception has occurred, use %tb to see the full traceback.

SystemExit: 2

To exit: use 'exit', 'quit', or Ctrl-D.
To exit: use 'exit', 'quit', or Ctrl-D.

Note: Most CLI users don’t care about the order of the command-line options. Hence, such fragile behavior should be avoided.