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236 changes: 64 additions & 172 deletions docs/arguments.rst
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Expand Up @@ -5,195 +5,118 @@ Arguments

.. currentmodule:: click

Arguments work similarly to :ref:`options <options>` but are positional.
They also only support a subset of the features of options due to their
syntactical nature. Click will also not attempt to document arguments for
you and wants you to :ref:`document them manually <documenting-arguments>`
in order to avoid ugly help pages.
Arguments are:

* Are positional in nature.
* Similar to a limited version of :ref:`options <options>` that can take an arbitrary number of inputs
* :ref:`Documented manually <documenting-arguments>`.

Useful and often used kwargs are:

* ``default``: Passes a default.
* ``nargs``: Sets the number of arguments. Set to -1 to take an arbitrary number.

Basic Arguments
---------------

The most basic option is a simple string argument of one value. If no
type is provided, the type of the default value is used, and if no default
value is provided, the type is assumed to be :data:`STRING`.
A minimal :class:`click.Argument` solely takes one string argument: the name of the argument. This will assume the argument is required, has no default, and is of the type ``str``.

Example:

.. click:example::
@click.command()
@click.argument('filename')
def touch(filename):
def touch(filename: str):
"""Print FILENAME."""
click.echo(filename)

And what it looks like:
And from the command line:

.. click:run::
invoke(touch, args=['foo.txt'])

Variadic Arguments
------------------

The second most common version is variadic arguments where a specific (or
unlimited) number of arguments is accepted. This can be controlled with
the ``nargs`` parameter. If it is set to ``-1``, then an unlimited number
of arguments is accepted.
An argument may be assigned a :ref:`parameter type <parameter-types>`. If no type is provided, the type of the default value is used. If no default value is provided, the type is assumed to be :data:`STRING`.

The value is then passed as a tuple. Note that only one argument can be
set to ``nargs=-1``, as it will eat up all arguments.
.. admonition:: Note on Required Arguments

Example:
It is possible to make an argument required by setting ``required=True``. It is not recommended since we think command line tools should gracefully degrade into becoming no ops. We think this because command line tools are often invoked with wildcard inputs and they should not error out if the wildcard is empty.

Multiple Arguments
-----------------------------------

To set the number of argument use the ``nargs`` kwarg. It can be set to any positive integer and -1. Setting it to -1, makes the number of arguments arbitrary (which is called variadic) and can only be used once. The arguments are then packed as a tuple and passed to the function.

.. click:example::
@click.command()
@click.argument('src', nargs=-1)
@click.argument('dst', nargs=1)
def copy(src, dst):
@click.argument('src', nargs=1)
@click.argument('dsts', nargs=-1)
def copy(src: str, dsts: tuple[str, ...]):
"""Move file SRC to DST."""
for fn in src:
click.echo(f"move {fn} to folder {dst}")
for destination in dsts:
click.echo(f"Copy {src} to folder {destination}")

And what it looks like:
And from the command line:

.. click:run::
invoke(copy, args=['foo.txt', 'bar.txt', 'my_folder'])

Note that this is not how you would write this application. The reason
for this is that in this particular example the arguments are defined as
strings. Filenames, however, are not strings! They might be on certain
operating systems, but not necessarily on all. For better ways to write
this, see the next sections.
invoke(copy, args=['foo.txt', 'usr/david/foo.txt', 'usr/mitsuko/foo.txt'])

.. admonition:: Note on Non-Empty Variadic Arguments
.. admonition:: Note on Handling Files

If you come from ``argparse``, you might be missing support for setting
``nargs`` to ``+`` to indicate that at least one argument is required.
This is not how you should handle files and files paths. This merely used as a simple example. See :ref:`handling-files` to learn more about how to handle files in parameters.

This is supported by setting ``required=True``. However, this should
not be used if you can avoid it as we believe scripts should gracefully
degrade into becoming noops if a variadic argument is empty. The
reason for this is that very often, scripts are invoked with wildcard
inputs from the command line and they should not error out if the
wildcard is empty.
Argument Escape Sequences
---------------------------

.. _file-args:
If you want to process arguments that look like options, like a file named ``-foo.txt`` or ``--foo.txt`` , you must pass the ``--`` separator first. After you pass the ``--``, you may only pass arguments. This is a common feature for POSIX command line tools.

File Arguments
--------------

Since all the examples have already worked with filenames, it makes sense
to explain how to deal with files properly. Command line tools are more
fun if they work with files the Unix way, which is to accept ``-`` as a
special file that refers to stdin/stdout.

Click supports this through the :class:`click.File` type which
intelligently handles files for you. It also deals with Unicode and bytes
correctly for all versions of Python so your script stays very portable.

Example:
Example usage:

.. click:example::
@click.command()
@click.argument('input', type=click.File('rb'))
@click.argument('output', type=click.File('wb'))
def inout(input, output):
"""Copy contents of INPUT to OUTPUT."""
while True:
chunk = input.read(1024)
if not chunk:
break
output.write(chunk)

And what it does:

.. click:run::
@click.argument('files', nargs=-1, type=click.Path())
def touch(files):
"""Print all FILES file names."""
for filename in files:
click.echo(filename)

with isolated_filesystem():
invoke(inout, args=['-', 'hello.txt'], input=['hello'],
terminate_input=True)
invoke(inout, args=['hello.txt', '-'])
And from the command line:

File Path Arguments
-------------------
.. click:run::
In the previous example, the files were opened immediately. But what if
we just want the filename? The naïve way is to use the default string
argument type. The :class:`Path` type has several checks available which raise nice
errors if they fail, such as existence. Filenames in these error messages are formatted
with :func:`format_filename`, so any undecodable bytes will be printed nicely.
invoke(touch, ['--', '-foo.txt', 'bar.txt'])

Example:
If you don't like the ``--`` marker, you can set ignore_unknown_options to True to avoid checking unknown options:

.. click:example::
@click.command()
@click.argument('filename', type=click.Path(exists=True))
def touch(filename):
"""Print FILENAME if the file exists."""
click.echo(click.format_filename(filename))
@click.command(context_settings={"ignore_unknown_options": True})
@click.argument('files', nargs=-1, type=click.Path())
def touch(files):
"""Print all FILES file names."""
for filename in files:
click.echo(filename)

And what it does:
And from the command line:

.. click:run::
with isolated_filesystem():
with open('hello.txt', 'w') as f:
f.write('Hello World!\n')
invoke(touch, args=['hello.txt'])
println()
invoke(touch, args=['missing.txt'])


File Opening Safety
-------------------

The :class:`FileType` type has one problem it needs to deal with, and that
is to decide when to open a file. The default behavior is to be
"intelligent" about it. What this means is that it will open stdin/stdout
and files opened for reading immediately. This will give the user direct
feedback when a file cannot be opened, but it will only open files
for writing the first time an IO operation is performed by automatically
wrapping the file in a special wrapper.

This behavior can be forced by passing ``lazy=True`` or ``lazy=False`` to
the constructor. If the file is opened lazily, it will fail its first IO
operation by raising an :exc:`FileError`.

Since files opened for writing will typically immediately empty the file,
the lazy mode should only be disabled if the developer is absolutely sure
that this is intended behavior.

Forcing lazy mode is also very useful to avoid resource handling
confusion. If a file is opened in lazy mode, it will receive a
``close_intelligently`` method that can help figure out if the file
needs closing or not. This is not needed for parameters, but is
necessary for manually prompting with the :func:`prompt` function as you
do not know if a stream like stdout was opened (which was already open
before) or a real file that needs closing.

Starting with Click 2.0, it is also possible to open files in atomic mode by
passing ``atomic=True``. In atomic mode, all writes go into a separate
file in the same folder, and upon completion, the file will be moved over to
the original location. This is useful if a file regularly read by other
users is modified.
invoke(touch, ['-foo.txt', 'bar.txt'])


.. _environment-variables:

Environment Variables
---------------------

Like options, arguments can also grab values from an environment variable.
Unlike options, however, this is only supported for explicitly named
environment variables.
Arguments can use environment variables. To do so, pass the name(s) of the environment variable(s) via `envvar` in ``click.argument``.

Example usage:
Checking one environment variable:

.. click:example::
Expand All @@ -208,59 +131,28 @@ And from the command line:
.. click:run::
with isolated_filesystem():
# Writing the file in the filesystem.
with open('hello.txt', 'w') as f:
f.write('Hello World!')
invoke(echo, env={'SRC': 'hello.txt'})

In that case, it can also be a list of different environment variables
where the first one is picked.

Generally, this feature is not recommended because it can cause the user
a lot of confusion.

Option-Like Arguments
---------------------

Sometimes, you want to process arguments that look like options. For
instance, imagine you have a file named ``-foo.txt``. If you pass this as
an argument in this manner, Click will treat it as an option.

To solve this, Click does what any POSIX style command line script does,
and that is to accept the string ``--`` as a separator for options and
arguments. After the ``--`` marker, all further parameters are accepted as
arguments.

Example usage:
Checking multiple environment variables:

.. click:example::
@click.command()
@click.argument('files', nargs=-1, type=click.Path())
def touch(files):
"""Print all FILES file names."""
for filename in files:
click.echo(filename)

And from the command line:

.. click:run::
invoke(touch, ['--', '-foo.txt', 'bar.txt'])

If you don't like the ``--`` marker, you can set ignore_unknown_options to
True to avoid checking unknown options:

.. click:example::
@click.command(context_settings={"ignore_unknown_options": True})
@click.argument('files', nargs=-1, type=click.Path())
def touch(files):
"""Print all FILES file names."""
for filename in files:
click.echo(filename)
@click.argument('src', envvar=['SRC', 'SRC_2'], type=click.File('r'))
def echo(src):
"""Print value of SRC environment variable."""
click.echo(src.read())

And from the command line:

.. click:run::
invoke(touch, ['-foo.txt', 'bar.txt'])
with isolated_filesystem():
# Writing the file in the filesystem.
with open('hello.txt', 'w') as f:
f.write('Hello World from second variable!')
invoke(echo, env={'SRC_2': 'hello.txt'})
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