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Parsing of command line options, yaml/jsonnet config files and/or environment variables based on argparse.

Project description


This package is an extension to python’s argparse which simplifies parsing of configuration options from command line arguments, json configuration files (yaml or jsonnet supersets), environment variables and hard-coded defaults.

The aim is similar to other projects such as configargparse, yconf, confuse, typer, OmegaConf, Fire and click. The obvious question is, why yet another package similar to many already existing ones? The answer is simply that none of the existing projects had the exact features we wanted and after analyzing the alternatives it seemed simpler to start a new project.


  • Parsers are configured just like with python’s argparse, thus it has a gentle learning curve.
  • Not exclusively intended for parsing command line arguments. The main focus is parsing configuration files and not necessarily from a command line tool.
  • Support for two popular supersets of json: yaml and jsonnet.
  • Support for nested namespaces which makes it possible to parse config files with non-flat hierarchies.
  • Three mechanisms to define parsers in a modular way: arguments from classes, methods and functions; sub-commands and parsers as arguments.
  • Parsing of relative paths within config files and path lists.
  • Several convenient action classes and types to ease common parsing use cases (paths, comparison operators, json schemas, enums …).
  • Support for command line tab argument completion using argcomplete.
  • Configuration values are overridden based on the following precedence.
    • Parsing command line: command line arguments (might include config file) > environment variables > default config file > defaults.
    • Parsing files: config file > environment variables > default config file > defaults.
    • Parsing environment: environment variables > default config file > defaults.


You can install using pip as:

pip install jsonargparse

By default the only dependency that jsonargparse installs is PyYAML. However, jsonargparse has several optional features that can be enabled by specifying any of the following extras requires: signatures, jsonschema, jsonnet, urls, argcomplete and reconplogger. There is also the all extras require that can be used to enable all optional features. Installing jsonargparse with extras require is as follows:

pip install "jsonargparse[signatures,urls]"  # Enable signatures and URLs features
pip install "jsonargparse[all]"              # Enable all optional features

The following table references sections that describe optional features and the corresponding extras requires that enables them.

  urls argcomplete jsonnet jsonschema signatures

Basic usage

There are multiple ways of using jsonargparse. The most simple way which requires to write the least amount of code is by using the .CLI function, for example:

from jsonargparse import CLI

def command(
    name: str,
    prize: int = 100
        name: Name of winner.
        prize: Amount won.
    print(f'{name} won {prize}€!')

if __name__ == '__main__':

Then in a shell you could run:

$ python Lucky --prize=1000
Lucky won 1000€!

.CLI without arguments searches for functions and classes defined in the same module and in the local context where .CLI is called. Giving a single or a list functions/classes as first argument to .CLI skips the automatic search and only includes what is given.

When .CLI receives a single class, the first arguments are used to instantiate the class, then a class method name must be given (i.e. methods become sub-commands) and the remaining arguments are used to run the class method. An example would be:

from random import randint
from jsonargparse import CLI

class Main:
    def __init__(
        max_prize: int = 100
            max_prize: Maximum prize that can be awarded.
        self.max_prize = max_prize

    def person(
        name: str
            name: Name of winner.
        return f'{name} won {randint(0, self.max_prize)}€!'

if __name__ == '__main__':

Then in a shell you could run:

$ python --max_prize=1000 person Lucky
Lucky won 632€!

If more than one function is given to .CLI, then any of them can be executed via sub-commands similar to the single class example above, i.e. function [arguments] where function is the name of the function to execute.

If multiple classes or a mixture of functions and classes is given to .CLI, to execute a method of a class, two levels of sub-commands are required. The first sub-command would be the name of the class and the second the name of the method, i.e. class [init_arguments] method [arguments]. For more details about the automatic adding of arguments from classes and functions and the use of configuration files refer to section classes-methods-functions.

This simple way of usage is similar and inspired by Fire. However, there are fundamental differences. First, the purpose is not allowing to call any python object from the command line. It is only intended for running functions and classes specifically written for this purpose. Second, the arguments are required to have type hints, and the values will be validated according to these. Third, the return values of the functions are not automatically printed. .CLI returns its value and it is up to the developer to decide what to do with it. Finally, jsonargparse has many features designed to help in creating convenient argument parsers such as: nested-namespaces, configuration-files, additional type hints (parsing-paths, restricted-numbers, restricted-strings) and much more.

The next section explains how to create an argument parser in a very low level argparse-style. However, as parsers get more complex, being able to define them in a modular way becomes important. Three mechanisms are available to define parsers in a modular way, see respective sections classes-methods-functions, sub-commands and parser-arguments.


An argument parser is created just like it is done with python’s argparse. You import the module, create a parser object and then add arguments to it. A simple example would be:

from jsonargparse import ArgumentParser
parser = ArgumentParser(
    description='Description for my app.')

    help='Help for option 1.')

    help='Help for option 2.')

After creating the parser, you can use it to parse command line arguments with the .ArgumentParser.parse_args function, after which you get an object with the parsed values or defaults available as attributes. For illustrative purposes giving to parse_args a list of arguments (instead of automatically getting them from the command line arguments), with the parser shown above you would observe:

>>> cfg = parser.parse_args(['--opt2', '2.3'])
>>> cfg.opt1, type(cfg.opt1)
(0, <class 'int'>)
>>> cfg.opt2, type(cfg.opt2)
(2.3, <class 'float'>)

If the parsing fails the standard behavior is that the usage is printed and the program is terminated. Alternatively you can initialize the parser with error_handler=None in which case a .ParserError is raised.

Nested namespaces

A difference with respect to the basic argparse is that it by using dot notation in the argument names, you can define a hierarchy of nested namespaces. So for example you could do the following:

>>> parser = ArgumentParser(prog='app')
>>> parser.add_argument('--lev1.opt1', default='from default 1')
>>> parser.add_argument('--lev1.opt2', default='from default 2')
>>> cfg = parser.get_defaults()
>>> cfg.lev1.opt1
'from default 2'
>>> cfg.lev1.opt2
'from default 2'

Configuration files

An important feature of jsonargparse is the parsing of yaml/json files. The dot notation hierarchy of the arguments (see nested-namespaces) are used for the expected structure in the config files.

The see :.ArgumentParser.default_config_files property can be set when creating a parser to specify patterns to search for configuration files. For example if a parser is created as ArgumentParser(default_config_files=['~/.myapp.yaml', '/etc/myapp.yaml']) when parsing if any of those two config files exist they will be parsed to override the defaults. Only the first matched config file is used. The default config file is always parsed first, this means that any command line arguments will override its values.

It is also possible to add an argument to explicitly provide a configuration file path. Providing a config file as an argument does not disable the parsing of default_config_files. The config argument would be parsed in the specific position among the command line arguments. Therefore the arguments found after would override the values from the config file. The config argument can be given multiple times, each overriding the values of the previous. Using the example parser from the nested-namespaces section above, we could have the following config file in yaml format:

# File: example.yaml
  opt1: from yaml 1
  opt2: from yaml 2

Then in python adding a config file argument and parsing some dummy arguments, the following would be observed:

>>> from jsonargparse import ArgumentParser, ActionConfigFile
>>> parser = ArgumentParser()
>>> parser.add_argument('--lev1.opt1', default='from default 1')
>>> parser.add_argument('--lev1.opt2', default='from default 2')
>>> parser.add_argument('--cfg', action=ActionConfigFile)
>>> cfg = parser.parse_args(['--lev1.opt1', 'from arg 1',
                             '--cfg', 'example.yaml',
                             '--lev1.opt2', 'from arg 2'])
>>> cfg.lev1.opt1
'from yaml 1'
>>> cfg.lev1.opt2
'from arg 2'

Instead of providing a path to a configuration file, a string with the configuration content can also be provided.

>>> cfg = parser.parse_args(['--cfg', '{"lev1":{"opt1":"from string 1"}}'])
>>> cfg.lev1.opt1
'from string 1'

The config file can also be provided as an environment variable as explained in section environment-variables. The configuration file environment variable is the first one to be parsed. So any argument provided through an environment variable would override the config file one.

A configuration file or string can also be parsed without parsing command line arguments. The methods for this are .ArgumentParser.parse_path and .ArgumentParser.parse_string to parse a config file or a config string respectively.

All parsers include a --print_config option. This is useful particularly for command line tools with a large set of options to create an initial config file including all default values. By default all entries are included, even the ones with null values. If this argument is given as --print_config=skip_null, then the entries with null values will be excluded.

Environment variables

The jsonargparse parsers can also get values from environment variables. The parser checks existing environment variables whose name is of the form [PREFIX_][LEV__]*OPT, that is all in upper case, first a prefix (set by env_prefix, or if unset the prog without extension) followed by underscore and then the argument name replacing dots with two underscores. Using the parser from the nested-namespaces section above, in your shell you would set the environment variables as:

export APP_LEV1__OPT1='from env 1'
export APP_LEV1__OPT2='from env 2'

Then in python the parser would use these variables, unless overridden by the command line arguments, that is:

>>> parser = ArgumentParser(env_prefix='APP', default_env=True)
>>> parser.add_argument('--lev1.opt1', default='from default 1')
>>> parser.add_argument('--lev1.opt2', default='from default 2')
>>> cfg = parser.parse_args(['--lev1.opt1', 'from arg 1'])
>>> cfg.lev1.opt1
'from arg 1'
>>> cfg.lev1.opt2
'from env 2'

Note that when creating the parser, default_env=True was given. By default .ArgumentParser.parse_args does not check environment variables, so it has to be enabled explicitly.

There is also the .ArgumentParser.parse_env function to only parse environment variables, which might be useful for some use cases in which there is no command line call involved.

If a parser includes an .ActionConfigFile argument, then the environment variable for this config file will be checked before all the other environment variables.

Classes, methods and functions

It is good practice to write python code in which parameters have type hints and are described in the docstrings. To make this well written code configurable, it wouldn’t make sense to duplicate information of types and parameter descriptions. To avoid this duplication, jsonargparse includes methods to automatically add their arguments: .SignatureArguments.add_class_arguments, .SignatureArguments.add_method_arguments and .SignatureArguments.add_function_arguments.

Take for example a class with its init and a method with docstrings as follows:

from typing import Dict, Union, List

class MyClass(MyBaseClass):
    def __init__(self, items: Dict[str, Union[int, List[int]]], **kwargs):
        """Initializer for MyClass.

            items: Description for items.

    def mymethod(self, value: float, flag: bool = False):
        """Description for mymethod.

            value: Description for value.
            flag: Description for flag.

Both MyClass and mymethod can easily be made configurable, the class initialized and the method executed as follows:

from jsonargparse import ArgumentParser, namespace_to_dict

parser = ArgumentParser()
parser.add_class_arguments(MyClass, 'myclass.init')
parser.add_method_arguments(MyClass, 'mymethod', 'myclass.method')

cfg = parser.parse_args()
myclass = MyClass(**namespace_to_dict(cfg.myclass.init))

The add_class_arguments call adds to the myclass.init key the items argument with description as in the docstring, it is set as required since it does not have a default value, and when parsed it is validated according to its type hint, i.e., a dict with values ints or list of ints. Also since the init has the **kwargs argument, the keyword arguments from MyBaseClass are also added to the parser. Similarly the add_method_arguments call adds to the myclass.method key the arguments value as a required float and flag as an optional boolean with default value false.

Instead of using namespace_to_dict to convert the namespaces to a dictionary, the .ArgumentParser object can be instantiated with parse_as_dict=True to get directly a dictionary from the parsing methods.

When parsing from a configuration file (see configuration-files) all the values can be given in a single config file. However, for convenience it is also possible that the values for each of the groups created by the calls to the add signature methods can be parsed from independent files. This means that for the example above there could be one general config file with contents:

  init: myclass.yaml
  method: mymethod.yaml

Then the files myclass.yaml and mymethod.yaml would only include the settings for each of the instantiation of the class and the call to the method respectively.

A wide range of type hints are supported. For exact details go to section type-hints. Some notes about the support for automatic adding of arguments are:

  • All positional arguments must have a type, otherwise the add arguments functions raise an exception.
  • Keyword arguments are ignored if they don’t have at least one type that is supported.
  • Recursive adding of arguments from base classes only considers the presence of *args and **kwargs. It does not check the code to identify if super().__init__ is called or with which arguments.

Since keyword arguments with unsupported types are ignored, during development it might be desired to know which arguments are ignored and the specific reason. This can be done by initializing .ArgumentParser with logger={'level': 'DEBUG'}. For more details about logging go to section logging.

For all features described above to work, two optional packages are required: jsonschema to support validation of complex type hints and docstring-parser to get the argument descriptions from the docstrings. Both these packages are included when jsonargparse is installed using the signatures extras require as explained in section installation.

Type hints

As explained in section classes-methods-functions type hints are required to automatically add arguments from signatures to a parser. Additional to this feature, a type hint can also be used independently when adding a single argument to the parser. For example, an argument that can be None or a float in the range (0, 1) or a positive int could be added using type hints as follows:

from typing import Optional, Union
from jsonargparse.typing import PositiveInt, OpenUnitInterval
parser.add_argument('--op', type=Optional[Union[PositiveInt, OpenUnitInterval]])

The support of type hints is designed to not require developers to change their types or default values. In other words, the idea is to support type hints whatever they may be, as opposed to requiring jsonargparse specific types. The types included in jsonargparse.typing are completely generic and could even be useful independent of the argument parsers.

A wide range of type hints are supported and with arbitrary complexity/nesting. Some notes about this support are:

  • Nested types are supported as long as at least one child type is supported.
  • Fully supported types are: str, bool, int, float, List, Iterable, Sequence, Any, Union, Optional, Enum, restricted types as explained in sections restricted-numbers and restricted-strings and paths and URLs as explained in sections parsing-paths and parsing-urls.
  • Dict is supported but only with str or int keys.
  • Tuple and Set are supported even though they can’t be represented in json distinguishable from a list. Each Tuple element position can have its own type and will be validated as such. In command line arguments, config files and environment variables, tuples and sets are represented as a list.
  • To set a value to None it is required to use null since this is how json/yaml defines it. To avoid confusion in the help, NoneType is displayed as null. For example function argument with type and default Optional[str] = None would be shown in the help as type: Union[str, null], default: null.

Class type and sub-classes

It is also possible to use an arbitrary class as a type such that the argument accepts this class or any derived subclass. In the config file or environment variable or command line argument, a class is represented by a dictionary with a class_path entry indicating the dot notation expression to import the class, and optionally some init_args that would be used to instantiate it. When parsing, it will be checked that the class can be imported, that it is a subclass of the given type and that init_args values correspond to valid arguments to instantiate it. After parsing, the config object will include the class_path and init_args entries. To get a config object with all sub-classes instantiated, the .ArgumentParser.instantiate_subclasses method is used.

A simple example would be having some config file config.yaml as:

  class_path: calendar.Calendar
    firstweekday: 1

Then in python:

>>> from calendar import Calendar
>>> parser = ArgumentParser(parse_as_dict=True)
>>> parser.add_argument('--calendar', type=Calendar)
>>> cfg = parser.parse_path('config.yaml')
>>> cfg['calendar']
{'class_path': 'calendar.Calendar', 'init_args': {'firstweekday': 1}}
>>> cfg = parser.instantiate_subclasses(cfg)
>>> cfg['calendar']
<calendar.Calendar object at 0x7ffa559aa940>

In this example the class_path points to the same class used for the type. But a subclass of Calendar with an extended list of init parameters would also work.

When using any of the methods described in classes-methods-functions, each argument with a class as the type can be given using a class_path and init_args pair.

There is also another method .SignatureArguments.add_subclass_arguments which does the same as add_argument in the example above, but has some added benefits: 1) the argument is added in a new group automatically; 2) the argument values can be given in an independent config file by specifying a path to it; 3) by default sets a useful metavar and help strings; and 4) a special --*.help argument is added that can be used to show the expected init_args details for a specific given class_path. Take for example a tool defined as:

from calendar import Calendar
from jsonargparse import ArgumentParser
parser = ArgumentParser()
parser.add_subclass_arguments(Calendar, 'calendar')
cfg = parser.parse_args()

If there is some subclass of Calendar which can be imported from mycode.MyCalendar, then it would be possible to see the corresponding init_args details by running the tool from the command line as:

python mycode.MyCalendar


A way to define parsers in a modular way is what in argparse is known as sub-commands. However, to promote modularity, in jsonargparse sub-commands work a bit different than in argparse. To add sub-commands to a parser, the .ArgumentParser.add_subcommands method is used. Then an existing parser is added as a sub-command using .add_subcommand. In a parsed config object the sub-command will be stored in the subcommand entry (or whatever dest was set to), and the values of the sub-command will be in an entry with the same name as the respective sub-command. An example of defining a parser with sub-commands is the following:

from jsonargparse import ArgumentParser
parser_subcomm1 = ArgumentParser()
parser_subcomm2 = ArgumentParser()
parser = ArgumentParser(prog='app')
subcommands = parser.add_subcommands()
subcommands.add_subcommand('subcomm1', parser_subcomm1)
subcommands.add_subcommand('subcomm2', parser_subcomm2)

Then some examples of parsing are the following:

>>> parser.parse_args(['subcomm1', '--op1', 'val1'])
Namespace(op0=None, subcomm1=Namespace(op1='val1'), subcommand='subcomm1')
>>> parser.parse_args(['--op0', 'val0', 'subcomm2', '--op2', 'val2'])
Namespace(op0='val0', subcomm2=Namespace(op2='val2'), subcommand='subcomm2')

Parsing config files with .ArgumentParser.parse_path or .ArgumentParser.parse_string is also possible. Though there can only be values for one of the sub-commands. The config file is not required to specify a value for subcommand. For the example parser above a valid yaml would be:

# File: example.yaml
op0: val0
  op1: val1

Parsing of environment variables works similar to .ActionParser. For the example parser above, all environment variables for subcomm1 would have as prefix APP_SUBCOMM1_ and likewise for subcomm2 as prefix APP_SUBCOMM2_. The sub-command to use could be chosen by setting environment variable APP_SUBCOMMAND.

It is possible to have multiple levels of sub-commands. With multiple levels there is one basic requirement: the sub-commands must be added in the order of the levels. This is, first call add_subcommands and add_subcommand for the first level. Only after do the same for the second level, and so on.

Json schemas

The .ActionJsonSchema class is provided to allow parsing and validation of values using a json schema. This class requires the jsonschema python package. Though note that jsonschema is not a requirement of the minimal jsonargparse install. To enable this functionality install with the jsonschema extras require as explained in section installation.

Check out the jsonschema documentation to learn how to write a schema. The current version of jsonargparse uses Draft7Validator. Parsing an argument using a json schema is done like in the following example:

>>> schema = {
...     "type" : "object",
...     "properties" : {
...         "price" : {"type" : "number"},
...         "name" : {"type" : "string"},
...     },
... }

>>> from jsonargparse import ActionJsonSchema
>>> parser.add_argument('--op', action=ActionJsonSchema(schema=schema))

>>> parser.parse_args(['--op', '{"price": 1.5, "name": "cookie"}'])
Namespace(op=Namespace(name='cookie', price=1.5))

Instead of giving a json string as argument value, it is also possible to provide a path to a json/yaml file, which would be loaded and validated against the schema. If the schema defines default values, these will be used by the parser to initialize the config values that are not specified. When adding an argument with the .ActionJsonSchema action, you can use “%s” in the help string so that in that position the schema is printed.

Jsonnet files

The Jsonnet support requires jsonschema and jsonnet python packages which are not included with minimal jsonargparse install. To enable this functionality install jsonargparse with the jsonnet extras require as explained in section installation.

By default an .ArgumentParser parses configuration files as yaml. However, if instantiated giving as argument parser_mode='jsonnet', then parse_args, parse_path and parse_string will expect config files to be in jsonnet format instead. Example:

>>> from jsonargparse import ArgumentParser, ActionConfigFile
>>> parser = ArgumentParser(parser_mode='jsonnet')
>>> parser.add_argument('--cfg', action=ActionConfigFile)
>>> cfg = parser.parse_args(['--cfg', 'example.jsonnet'])

Jsonnet files are commonly parametrized, thus requiring external variables for parsing. For these cases, instead of changing the parser mode away from yaml, the .ActionJsonnet class can be used. This action allows to define an argument which would be a jsonnet string or a path to a jsonnet file. Moreover, another argument can be specified as the source for any external variables required, which would be either a path to or a string containing a json dictionary of variables. Its use would be as follows:

from jsonargparse import ArgumentParser, ActionJsonnet, ActionJsonnetExtVars
parser = ArgumentParser()

For example, if a jsonnet file required some external variable param, then the jsonnet and the external variable could be given as:

cfg = parser.parse_args(['--in_ext_vars', '{"param": 123}',
                         '--in_jsonnet', 'path_to_jsonnet'])

Note that the external variables argument must be provided before the jsonnet path so that this dictionary already exists when parsing the jsonnet.

The .ActionJsonnet class also accepts as argument a json schema, in which case the jsonnet would be validated against this schema right after parsing.

Parsing paths

For some use cases it is necessary to parse file paths, checking its existence and access permissions, but not necessarily opening the file. Moreover, a file path could be included in a config file as relative with respect to the config file’s location. After parsing it should be easy to access the parsed file path without having to consider the location of the config file. To help in these situations jsonargparse includes a type generator .path_type, some predefined types (e.g. .Path_fr) and the .ActionPath and .ActionPathList classes.

For example suppose you have a directory with a configuration file app/config.yaml and some data app/data/info.db. The contents of the yaml file is the following:

# File: config.yaml
  info: data/info.db

To create a parser that checks that the value of is a file that exists and is readable, the following could be done:

from jsonargparse import ArgumentParser
from jsonargparse.typing import Path_fr
parser = ArgumentParser()
parser.add_argument('', type=Path_fr)
cfg = parser.parse_path('app/config.yaml')

The fr in the type are flags stand for file and readable. After parsing the value of will be an instance of the .Path class that allows to get both the original relative path as included in the yaml file, or the corresponding absolute path:

>>> str(

Likewise directories can be parsed for instance using the .Path_dw type, which would require a directory to exist and be writeable. New path types can be created using the .path_type function. For example to create a type for files that must exist and be both readable and writeable, the command would be Path_frw = path_type('frw'). If the file app/config.yaml is not writeable, then usig the type to cast Path_frw('app/config.yaml') would raise a TypeError: File is not writeable exception. For more information of all the mode flags supported, refer to the documentation of the .Path class.

The content of a file that a .Path instance references can be read by using the .Path.get_content method. For the previous example would be info_db =

Adding arguments with path types is equivalent to adding using for example action=ActionPath(mode='fr') instead of a type=Path_fr. However, the type option is preferred.

An argument with a path type can be given nargs='+' to parse multiple paths. But it might also be wanted to parse a list of paths found in a plain text file or from stdin. For this the .ActionPathList is used and as argument either the path to a file listing the paths is given or the special '-' string for reading the list from stdin. For for example:

from jsonargparse import ActionPathList
parser.add_argument('--list', action=ActionPathList(mode='fr'))
cfg = parser.parse_args(['--list', 'paths.lst')  # Text file with paths
cfg = parser.parse_args(['--list', '-')          # List from stdin

If nargs='+' is given to add_argument with .ActionPathList then a single list is generated including all paths in all provided lists.

Note: the .Path class is currently not fully supported in windows.

Parsing URLs

The .path_type function also supports URLs which after parsing the .Path.get_content method can be used to perform a GET request to the corresponding URL and retrieve its content. For this to work the validators and requests python packages are required which will be installed along with jsonargparse if installed with the urls extras require as explained in section installation.

The 'u' flag is used to parse URLs. For example if it is desired that an argument can be either a readable file or URL, the type would be created as Path_fur = path_type('fur'). If the value appears to be a URL according to validators.url.url then a HEAD request would be triggered to check if it is accessible, and if so, the parsing succeeds. To get the content of the parsed path, without needing to care if it is a local file or a URL, the .Path.get_content can be used.

If after importing jsonargparse you run jsonargparse.set_url_support(True), the following functions and classes will also support loading from URLs: .ArgumentParser.parse_path, .ArgumentParser.get_defaults (default_config_files argument), .ActionConfigFile, .ActionJsonSchema, .ActionJsonnet and .ActionParser. This means for example that a tool that can receive a configuration file via .ActionConfigFile is able to get the config file from a URL, thus something like the following would work:

$ --cfg

Restricted numbers

It is quite common that when parsing a number, its range should be limited. To ease these cases the module jsonargparse.typing includes some predefined types and a function .restricted_number_type to define new types. The predefined types are: .PositiveInt, .NonNegativeInt, .PositiveFloat, .NonNegativeFloat, .ClosedUnitInterval and .OpenUnitInterval. Examples of usage are:

from jsonargparse.typing import PositiveInt, PositiveFloat, restricted_number_type
# float larger than zero
parser.add_argument('--op1', type=PositiveFloat)
# between 0 and 10
from_0_to_10 = restricted_number_type('from_0_to_10', int, [('>=', 0), ('<=', 10)])
parser.add_argument('--op2', type=from_0_to_10)
# either int larger than zero or 'off' string
def int_or_off(x): return x if x == 'off' else PositiveInt(x)
parser.add_argument('--op3', type=int_or_off))

Restricted strings

Similar to the restricted numbers, there is a function to create string types that are restricted to match a given regular expression: .restricted_string_type. A predefined type is .Email which is restricted so that it follows the normal email pattern. For example to add an argument required to be exactly four uppercase letters:

from jsonargparse.typing import Email, restricted_string_type
CodeType = restricted_string_type('CodeType', '^[A-Z]{4}$')
parser.add_argument('--code', type=CodeType)
parser.add_argument('--email', type=Email)

Enum arguments

Another case of restricted values is string choices. In addition to the common choices given as a list of strings, it is also possible to provide as type an Enum class. This has the added benefit that strings are mapped to some desired values. For example:

>>> class MyEnum(enum.Enum):
...     choice1 = -1
...     choice2 = 0
...     choice3 = 1
>>> parser.add_argument('--op', type=MyEnum)
>>> parser.parse_args(['--op=choice1'])
Namespace(op=<MyEnum.choice1: -1>)

Boolean arguments

Parsing boolean arguments is very common, however, the original argparse only has a limited support for them, via store_true and store_false. Futhermore unexperienced users might mistakenly use type=bool which would not provide the intended behavior.

With jsonargparse adding an argument with type=bool the intended action is implemented. If given as values {'yes', 'true'} or {'no', 'false'} the corresponding parsed values would be True or False. For example:

>>> parser.add_argument('--op1', type=bool, default=False)
>>> parser.add_argument('--op2', type=bool, default=True)
>>> parser.parse_args(['--op1', 'yes', '--op2', 'false'])
Namespace(op1=True, op2=False)

To use type=bool jsonargparse needs to be installed with the jsonschema extras require as explained in section installation.

Sometimes it is also useful to define two paired options, one to set True and the other to set False. The .ActionYesNo class makes this straightforward. A couple of examples would be:

from jsonargparse import ActionYesNo
# --opt1 for true and --no_opt1 for false.
parser.add_argument('--op1', action=ActionYesNo)
# --with-opt2 for true and --without-opt2 for false.
parser.add_argument('--with-op2', action=ActionYesNo(yes_prefix='with-', no_prefix='without-'))

If the .ActionYesNo class is used in conjunction with nargs='?' the options can also be set by giving as value any of {'true', 'yes', 'false', 'no'}. .ActionYesNo works without any extras require.

Parsers as arguments

Sometimes it is useful to take an already existing parser that is required standalone in some part of the code, and reuse it to parse an inner node of another more complex parser. For these cases an argument can be defined using the .ActionParser class. An example of how to use this class is the following:

from jsonargparse import ArgumentParser, ActionParser
inner_parser = ArgumentParser(prog='app1')
outer_parser = ArgumentParser(prog='app2')

When using the .ActionParser class, the value of the node in a config file can be either the complex node itself, or the path to a file which will be loaded and parsed with the corresponding inner parser. Naturally using .ActionConfigFile to parse a complete config file will parse the inner nodes correctly.

From the command line the help of the inner parsers can be shown by calling the tool with a prefixed help command, that is, for the example above it would be

Regarding environment variables, the prefix of the outer parser will be used to populate the leaf nodes of the inner parser. In the example above, if inner_parser is used to parse environment variables, then as normal APP1_OP1 would be checked to populate option op1. But if outer_parser is used, then APP2_INNER__NODE__OP1 would be checked to populate inner.node.op1.

An important detail to note is that the parsers that are given to .ActionParser are internally modified. Therefore, to use the parser both as standalone and an inner node, it is necessary to implement a function that instantiates the parser. This function would be used in one place to get an instance of the parser for standalone parsing, and in some other place use the function to provide an instance of the parser to .ActionParser.

Tab completion

Tab completion is available for jsonargparse parsers by using the argcomplete package. There is no need to implement completer functions or to call argcomplete.autocomplete since this is done automatically by .ArgumentParser.parse_args. The only requirement to enable tab completion is to install argcomplete either directly or by installing jsonargparse with the argcomplete extras require as explained in section installation. Then the tab completion can be enabled globally for all argcomplete compatible tools or for each individual tool. A simple tool would be:

#!/usr/bin/env python3

from typing import Optional
from jsonargparse import ArgumentParser

parser = ArgumentParser()
parser.add_argument('--bool', type=Optional[bool])


Then in a bash shell you can add the executable bit to the script, activate tab completion and use it as follows:

$ chmod +x
$ eval "$(register-python-argcomplete"

$ ./ --bool <TAB><TAB>
false  null   true
$ ./ --bool f<TAB>
$ ./ --bool false


The parsers from jsonargparse log some basic events, though by default this is disabled. To enable it the logger argument should be set when creating an .ArgumentParser object. The intended use is to give as value an already existing logger object which is used for the whole application. Though for convenience to enable a default logger the logger argument can also receive True or a string which sets the name of the logger or a dictionary that can include the name and the level, e.g. {"name": "myapp", "level": "ERROR"}. If reconplogger is installed, setting logger to True or a dictionary without specifying a name, then the reconplogger is used.


Contributions to the jsonargparse package are very welcome, be it just to create issues for reporting bugs and proposing enhancements, or more directly by creating pull requests.

If you intend to work with the source code, note that this project does not include any requirements.txt file. This is by intention. To make it very clear what are the requirements for different use cases, all the requirements of the project are stored in the file setup.cfg. The basic runtime requirements are defined in section [options] in the install_requires entry. All extras requires for optional features listed in installation are stored in section [options.extras_require]. Also there are test, test_no_urls, dev and doc entries in the same [options.extras_require] section which lists requirements for testing, development and documentation building.

The recommended way to work with the source code is the following. First clone the repository, then create a virtual environment, activate it and finally install the development requirements. More precisely the steps are:

git clone
cd jsonargparse
virtualenv -p python3 venv
. venv/bin/activate

The crucial step is installing the requirements which would be done by running:

pip install -e ".[dev,all]"

Running the unit tests can be done either using using tox or the script. The unit tests are also installed with the package, thus can be used to in a production system.

tox  # Run tests using tox
./ test_coverage  # Run tests and generate coverage report
python3 -m jsonargparse_tests  # Run tests for installed package

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