using-cli.md

Using a Fire CLI

Basic usage

Every Fire command corresponds to a Python component.

The simplest Fire command consists of running your program with no additional arguments. This command corresponds to the Python component you called the Fire function on. If you did not supply an object in the call to Fire, then the context in which Fire was called will be used as the Python component.

You can append -- --help to any command to see what Python component it corresponds to, as well as the various ways in which you can extend the command. Flags are always separated from the Fire command by an isolated -- in order to distinguish between flags and named arguments.

Given a Fire command that corresponds to a Python object, you can extend that command to access a member of that object, call it with arguments if it is a function, instantiate it if it is a class, or index into it if it is a list.

Read on to learn about how you can write a Fire command corresponding to whatever Python component you're looking for.

Accessing members of an object

If your command corresponds to an object, you can extend your command by adding the name of a member of that object as a new argument to the command. The resulting command will correspond to that member.

For example, if the object your command corresponds to has a method defined on it named 'whack', then you can add the argument 'whack' to your command, and the resulting new command corresponds to the whack method.

As another example, if the object your command corresponds to has a property named high_score, then you can add the argument 'high-score' to your command, and the resulting new command corresponds to the value of the high_score property.

Accessing members of a dict

If your command corresponds to a dict, you can extend your command by adding the name of one of the dict's keys as an argument.

For example, widget function-that-returns-dict key will correspond to the value of the item with key key in the dict returned by function_that_returns_dict.

Accessing members of a list or tuple

If your command corresponds to a list or tuple, you can extend your command by adding the index of an element of the component to your command as an argument.

For example, widget function-that-returns-list 2 will correspond to item 2 of the result of function_that_returns_list.

Calling a function

If your command corresponds to a function, you can extend your command by adding the arguments of this function. Arguments can be specified positionally, or by name. To specify an argument by name, use flag syntax.

For example, suppose your command corresponds to the function double:

def double(value=0):
  return 2 * value

Then you can extend your command using named arguments as command --value 5, or using positional arguments as command 5. In both cases, the new command corresponds to the result of the function, in this case the number 10.

You can force a function that takes a variable number of arguments to be evaluated by adding a separator (the default separator is the hyphen, "-"). This will prevent arguments to the right of the separator from being consumed for calling the function. This is useful if the function has arguments with default values, or if the function accepts *varargs, or if the function accepts **kwargs.

See also the section on Changing the Separator.

Instantiating a class

If your command corresponds to a class, you can extend your command by adding the arguments of the class's __init__ function. Arguments must be specified by name, using the flags syntax. See the section on calling a function for more details.

Similarly, when passing arguments to a callable object (an object with a custom __call__ function), those arguments must be passed using flags syntax.

Using Flags with Fire CLIs

Command line arguments to a Fire CLI are normally consumed by Fire, as described in the Basic Usage section. In order to set Flags, put the flags after the final standalone -- argument. (If there is no -- argument, then no arguments are used for flags.)

For example, to set the alsologtostderr flag, you could run the command: widget bang --noise=boom -- --alsologtostderr. The --noise argument is consumed by Fire, but the --alsologtostderr argument is treated as a normal Flag.

All CLIs built with Python Fire share some flags, as described in the next sections.

Python Fire's Flags

As described in the Using Flags section, you must add an isolated -- argument in order to have arguments treated as Flags rather than be consumed by Python Fire. All arguments to a Fire CLI after the final standalone -- argument are treated as Flags.

The following flags are accepted by all Fire CLIs: --interactive/-i, --help/-h, --separator, --completion, --trace, and --verbose/-v, as described in the following sections.

--interactive: Interactive mode

Call widget -- --interactive or widget -- -i to enter interactive mode. This will put you in an IPython REPL, with the variable widget already defined.

You can then explore the Python object that widget corresponds to interactively using Python.

--completion: Generating a completion script

Call widget -- --completion to generate a completion script for the Fire CLI widget. To save the completion script to your home directory, you could e.g. run widget -- --completion > ~/.widget-completion. You should then source this file; to get permanent completion, source this file from your .bashrc file.

Call widget -- --completion fish to generate a completion script for the Fish shell. Source this file from your fish.config.

If the commands available in the Fire CLI change, you'll have to regenerate the completion script and source it again.

--help: Getting help

Let say you have a command line tool named widget that was made with Fire. How do you use this Fire CLI?

The simplest way to get started is to run widget -- --help. This will give you usage information for your CLI. You can always append -- --help to any Fire command in order to get usage information for that command and any subcommands.

Additionally, help will be displayed if you hit an error using Fire. For example, if you try to pass too many or too few arguments to a function, then help will be displayed. Similarly, if you try to access a member that does not exist, or if you index into a list with too high an index, then help will be displayed.

The displayed help shows information about which Python component your command corresponds to, as well as usage information for how to extend that command.

--trace: Getting a Fire trace

In order to understand what is happening when you call Python Fire, it can be useful to request a trace. This is done via the --trace flag, e.g. widget whack 5 -- --trace.

A trace provides step by step information about how the Fire command was executed. In includes which actions were taken, starting with the initial component, leading to the final component represented by the command.

A trace is also shown alongside the help if your Fire command reaches an error.

--separator: Changing the separator

As described in Calling a Function, you can use a separator argument when writing a command that corresponds to calling a function. The separator will cause the function to be evaluated or the class to be instantiated using only the arguments left of the separator. Arguments right of the separator will then be applied to the result of the function call or to the instantiated object.

The default separator is -.

If you want to supply the string "-" as an argument, then you will have to change the separator. You can choose a new separator by supplying the --separator flag to Fire.

Here's an example to demonstrate separator usage. Let's say you have a function that takes a variable number of args, and you want to call that function, and then upper case the result. Here's how to do it:

# Here's the Python function
def display(arg1, arg2='!'):
  return arg1 + arg2

# Here's what you can do from Bash (Note: the default separator is the hyphen -)
display hello                         # hello!
display hello upper                   # helloupper
display hello - upper                 # HELLO!
display - SEP upper -- --separator SEP    # -!

Notice how in the third and fourth lines, the separator caused the display function to be called with the default value for arg2. In the fourth example, we change the separator to the string "SEP" so that we can pass '-' as an argument.

--verbose: Verbose usage

Adding the -v or --verbose flag turns on verbose mode. This will eg reveal private members in the usage string. Often these members will not actually be usable from the command line tool. As such, verbose mode should be considered a debugging tool, but not fully supported yet.