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How do I parse command-line arguments in Python?
Start with an ArgumentParser, describe each input, and parse the arguments. This small script accepts two required integers and an optional flag:
import argparse
parser = argparse.ArgumentParser(description="Add two integers.")
parser.add_argument("left", type=int, help="first integer")
parser.add_argument("right", type=int, help="second integer")
parser.add_argument("--verbose", action="store_true", help="show a labeled result")
args = parser.parse_args()
result = args.left + args.right
print(f"{args.left} + {args.right} = {result}" if args.verbose else result)Save it as add.py and run it from a terminal:
python add.py 3 8
python add.py 3 8 --verbose
python add.py --helpThe first command prints 11; the second prints 3 + 8 = 11. The help option displays usage generated from the parser definition. The official Python Argparse Tutorial calls argparse the recommended command-line parsing module in the standard library.
What happens when you call parse_args()?
add_argument() describes how command-line tokens should be interpreted; parse_args() reads and validates those tokens. With no argument passed to the method, it reads the command-line arguments from sys.argv. The parsed values are returned as attributes on a Namespace, such as args.left and args.verbose. See the argparse API reference.
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You can instead supply a list of strings. This is useful for testing parsing behavior without launching the script as a separate process:
args = parser.parse_args(["3", "8", "--verbose"])
print(args.left, args.right, args.verbose)Because left and right were declared with type=int, the two values in the namespace are integers, not strings. The API reference and the Python command-line documentation explain the relationship between the interpreter’s command line and the arguments available to a program.
How do I add positional arguments and options?
A positional argument is identified by its place in the command, while an option is introduced by a flag such as --output. Choose names and help text that make the command understandable to someone who has not read the source.
Required positional values
A bare name creates a required positional argument. For example, parser.add_argument("filename") makes the script expect a filename after the script name. Positional values are appropriate when the command has a natural order and the values are essential to its main task.
Named options and defaults
Use one or more option strings to create a flag or a value-taking option. A short and long spelling can refer to the same value:
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parser.add_argument("-o", "--output", default="result.txt",
help="path for the output file")If the user omits this option, the value is result.txt. The default is also available as an attribute on the parsed namespace. Use required=True when an option-style value must be supplied, though a required positional is often clearer for a core input.
Convert and constrain values
Set type to convert a supplied value, such as type=int. Use choices to limit accepted values to a known set:
parser.add_argument("--format", choices=["json", "csv"], default="json")If a user supplies an invalid integer or a value outside the declared choices, the parser reports an error rather than leaving your program to handle an unchecked string. For more complex validation or conversion, define the behavior deliberately rather than silently accepting a value your program cannot use.
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Boolean on/off flags
For a flag that is false when omitted and true when present, use action="store_true". In the opening example, --verbose works this way. This is clearer than asking a user to type a value such as --verbose true.
Count repeated flags
Use action="count" when each repetition should increase a counter, for example to support -v and -vv:
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parser.add_argument("-v", "--verbose", action="count", default=0,
help="increase verbosity; repeat for more detail")Then branch on args.verbose to choose the amount of diagnostic output. Set a default explicitly when your code expects a numeric count even if the option is absent.
Arguments that consume multiple values
Use nargs to specify how many values an argument consumes or to express a variable number of values. For example, nargs=2 accepts exactly two values, while nargs="+" accepts one or more. Select a form that makes the command’s expected input unambiguous; the returned value for a multi-value argument is a sequence rather than a single scalar.
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Options that cannot be combined
When two choices are incompatible, define a mutually exclusive group so the parser can reject a command that supplies both:
group = parser.add_mutually_exclusive_group()
group.add_argument("--quiet", action="store_true")
group.add_argument("--verbose", action="store_true")Add required=True to the group only if the user must choose one of its options. The official Argparse Tutorial covers actions, nargs, and mutually exclusive groups.
How do I display help and handle invalid input?
Give the parser a description and each argument a help string. Running the program with --help (or -h) displays generated usage and option descriptions. You usually do not need to implement a separate help screen for a straightforward command-line tool.
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If the user omits a required positional argument, supplies a value that cannot be converted by type, or uses an unrecognized argument, argparse prints usage and an error message. This behavior helps users correct invocation mistakes. Keep help text concrete: explain what a value controls, its expected format, and any meaningful default.
What if a positional value starts with a hyphen?
A value such as -f may look like an option. Put -- before it to indicate that subsequent tokens should be treated as positional input:
python my_script.py -- -fThe tutorial demonstrates the same delimiter with parse_args(["--", "-f"]). This is particularly useful when a filename or other positional value begins with a hyphen and could otherwise be interpreted as an option.
How do I choose between argparse, optparse, and getopt?
| Need | Suitable choice | Guidance |
|---|---|---|
| A typical new script or command-line tool | argparse |
The standard-library recommendation for general use, with positional and optional arguments, conversion, validation, help, and subcommands. Python Argparse Tutorial |
| An established program already built around older option parsing | optparse or a planned migration |
Check compatibility and behavior before changing an existing command interface. Python command-line libraries |
| C-style option processing or a deliberately low-level interface | getopt |
Python documents it as a C-style parser and provides an argparse equivalent. Python getopt documentation |
For new general-purpose interfaces, begin with argparse. Do not change an established tool solely for stylistic reasons: compare interface compatibility and the behavior your users and scripts depend on. The argparse API reference describes differences from optparse, including positional arguments and subcommands.
How can I test argument parsing?
Separate parsing from the rest of the program when the parser needs to be exercised by tests or called by other Python code. One simple pattern is to define a function that accepts an explicit argument list:
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import argparse
def parse_arguments(argv=None):
parser = argparse.ArgumentParser(description="Add two integers.")
parser.add_argument("left", type=int)
parser.add_argument("right", type=int)
parser.add_argument("--verbose", action="store_true")
return parser.parse_args(argv)
if __name__ == "__main__":
args = parse_arguments()
total = args.left + args.right
print(f"{args.left} + {args.right} = {total}" if args.verbose else total)When called without a list, parse_arguments() retains the usual behavior of parsing the process command line. A test can pass a fixed list such as ["3", "8", "--verbose"], inspect the returned attributes, and verify the conversion and flag behavior without depending on a user’s terminal invocation.
Troubleshooting common argparse problems
- “The following arguments are required”: Supply the missing positional input, or revise the declaration if it was meant to be optional. A bare positional name is required by default.
- “Invalid int value” or a similar conversion error: Check that the token matches the declared
type. If the input is not meant to be an integer, remove or change that conversion deliberately. - “Unrecognized arguments”: Check spelling and whether a value-taking option has its value. If a token is intended as a hyphen-leading positional value, place
--before it. - An option is unexpectedly false or missing: Confirm the invocation includes the flag and that the code reads the correct namespace attribute. With
store_true, omission means false. - Two options conflict: If they are mutually exclusive, provide only one. If they should be compatible, do not put them in the same mutually exclusive group.
- Tests read the real process arguments: Pass a list to
parse_args()or to a wrapper function acceptingargv; no-argument parsing readssys.argv.
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Frequently Asked Questions
Does argparse come with Python?
Yes. It is part of Python’s standard library, so a typical installation does not require a separate package install.
What does parse_args() return?
It returns an argparse Namespace whose attributes correspond to the arguments declared on the parser.
Can I use argparse inside a notebook or another Python function?
Yes. Pass a list of strings to parse_args() when you want to parse a controlled set of arguments rather than the current process command line.
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