Parameters

Click supports two types of parameters for scripts: options and arguments. There is generally some confusion among authors of command line scripts of when to use which, so here is a quick overview of the differences. As its name indicates, an option is optional. While arguments can be optional within reason, they are much more restricted in how optional they can be.

To help you decide between options and arguments, the recommendation is to use arguments exclusively for things like going to subcommands or input filenames / URLs, and have everything else be an option instead.

Differences

Arguments can do less than options. The following features are only available for options:

  • automatic prompting for missing input
  • act as flags (boolean or otherwise)
  • option values can be pulled from environment variables, arguments can not
  • options are fully documented in the help page, arguments are not (this is intentional as arguments might be too specific to be automatically documented)

On the other hand arguments, unlike options, can accept an arbitrary number of arguments. Options can strictly ever only accept a fixed number of arguments (defaults to 1).

Parameter Types

Parameters can be of different types. Types can be implemented with different behavior and some are supported out of the box:

str / click.STRING:
The default parameter type which indicates unicode strings.
int / click.INT:
A parameter that only accepts integers.
float / click.FLOAT:
A parameter that only accepts floating point values.
bool / click.BOOL:
A parameter that accepts boolean values. This is automatically used for boolean flags. If used with string values 1, yes, y and true convert to True and 0, no, n and false convert to False.
click.UUID:
A parameter that accepts UUID values. This is not automatically guessed but represented as uuid.UUID.
class click.File(mode='r', encoding=None, errors='strict', lazy=None, atomic=False)

Declares a parameter to be a file for reading or writing. The file is automatically closed once the context tears down (after the command finished working).

Files can be opened for reading or writing. The special value - indicates stdin or stdout depending on the mode.

By default, the file is opened for reading text data, but it can also be opened in binary mode or for writing. The encoding parameter can be used to force a specific encoding.

The lazy flag controls if the file should be opened immediately or upon first IO. The default is to be non lazy for standard input and output streams as well as files opened for reading, lazy otherwise.

Starting with Click 2.0, files can also be opened atomically in which case 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.

See File Arguments for more information.

class click.Path(exists=False, file_okay=True, dir_okay=True, writable=False, readable=True, resolve_path=False)

The path type is similar to the File type but it performs different checks. First of all, instead of returning an open file handle it returns just the filename. Secondly, it can perform various basic checks about what the file or directory should be.

Parameters:
  • exists – if set to true, the file or directory needs to exist for this value to be valid. If this is not required and a file does indeed not exist, then all further checks are silently skipped.
  • file_okay – controls if a file is a possible value.
  • dir_okay – controls if a directory is a possible value.
  • writable – if true, a writable check is performed.
  • readable – if true, a readable check is performed.
  • resolve_path – if this is true, then the path is fully resolved before the value is passed onwards. This means that it’s absolute and symlinks are resolved.
class click.Choice(choices)

The choice type allows a value to be checked against a fixed set of supported values. All of these values have to be strings.

See Choice Options for an example.

class click.IntRange(min=None, max=None, clamp=False)

A parameter that works similar to click.INT but restricts the value to fit into a range. The default behavior is to fail if the value falls outside the range, but it can also be silently clamped between the two edges.

See Range Options for an example.

Custom parameter types can be implemented by subclassing click.ParamType. For simple cases, passing a Python function that fails with a ValueError is also supported, though discouraged.

Parameter Names

Parameters (both options and arguments) accept a number of positional arguments which are the parameter declarations. Each string with a single dash is added as short argument; each string starting with a double dash as long one. If a string is added without any dashes, it becomes the internal parameter name which is also used as variable name.

If a parameter is not given a name without dashes, a name is generated automatically by taking the longest argument and converting all dashes to underscores. For an option with ('-f', '--foo-bar'), the parameter name is foo_bar. For an option with ('-x',), the parameter is x. For an option with ('-f', '--filename', 'dest'), the parameter is called dest.

Implementing Custom Types

To implement a custom type, you need to subclass the ParamType class. Types can be invoked with or without context and parameter object, which is why they need to be able to deal with this.

The following code implements an integer type that accepts hex and octal numbers in addition to normal integers, and converts them into regular integers:

import click

class BasedIntParamType(click.ParamType):
    name = 'integer'

    def convert(self, value, param, ctx):
        try:
            if value[:2].lower() == '0x':
                return int(value[2:], 16)
            elif value[:1] == '0':
                return int(value, 8)
            return int(value, 10)
        except ValueError:
            self.fail('%s is not a valid integer' % value, param, ctx)

BASED_INT = BasedIntParamType()

As you can see, a subclass needs to implement the ParamType.convert() method and optionally provide the ParamType.name attribute. The latter can be used for documentation purposes.