Testing Click Applications

Click provides the click.testing module to help you invoke command line applications and check their behavior.

Warning

These tools should only be used for testing since they change the entire interpreter state for simplicity. They are not thread-safe: see Running tests in parallel for the supported way to run CLI tests concurrently.

The examples use pytest style tests.

Basic Example

The key pieces are:

• CliRunner - used to invoke commands as command line scripts.

• Result - returned from CliRunner.invoke(). Captures output data, exit code, optional exception, and captures the output as bytes and binary data.

hello.py

import click

@click.command()
@click.argument('name')
def hello(name):
   click.echo(f'Hello {name}!')

test_hello.py

from click.testing import CliRunner
from hello import hello

def test_hello_world():
  runner = CliRunner()
  result = runner.invoke(hello, ['Peter'])
  assert result.exit_code == 0
  assert result.output == 'Hello Peter!\n'

Subcommands

A subcommand name must be specified in the args parameter CliRunner.invoke():

sync.py

import click

@click.group()
@click.option('--debug/--no-debug', default=False)
def cli(debug):
   click.echo(f"Debug mode is {'on' if debug else 'off'}")

@cli.command()
def sync():
   click.echo('Syncing')

test_sync.py

from click.testing import CliRunner
from sync import cli

def test_sync():
  runner = CliRunner()
  result = runner.invoke(cli, ['--debug', 'sync'])
  assert result.exit_code == 0
  assert 'Debug mode is on' in result.output
  assert 'Syncing' in result.output

Context Settings

Additional keyword arguments passed to CliRunner.invoke() will be used to construct the initial Context object. For example, setting a fixed terminal width equal to 60:

sync.py

import click

@click.group()
def cli():
   pass

@cli.command()
def sync():
   click.echo('Syncing')

test_sync.py

from click.testing import CliRunner
from sync import cli

def test_sync():
  runner = CliRunner()
  result = runner.invoke(cli, ['sync'], terminal_width=60)
  assert result.exit_code == 0
  assert 'Debug mode is on' in result.output
  assert 'Syncing' in result.output

File System Isolation

To test a command that reads or writes files, give each test its own directory and pass absolute paths to the command. Pytest’s tmp_path fixture provides a fresh directory per test; tempfile.TemporaryDirectory works without pytest.

cat.py

import click

@click.command()
@click.argument('f', type=click.File())
def cat(f):
   click.echo(f.read())

test_cat.py

from click.testing import CliRunner
from cat import cat

def test_cat(tmp_path):
   hello = tmp_path / 'hello.txt'
   hello.write_text('Hello World!')

   runner = CliRunner()
   result = runner.invoke(cat, [str(hello)])
   assert result.exit_code == 0
   assert result.output == 'Hello World!\n'

Because the directory is unique to the test and the command receives an absolute path, nothing leaks between tests and the working directory is never changed.

Running tests in parallel

Isolating the file system with pytest is thread-safe: each test, or each thread, works in its own directory through absolute paths, with no shared state.

CliRunner.invoke() is different. While a command runs it swaps several pieces of process-global state into place (like sys.stdout, sys.stderr, sys.stdin, etc). Two invocations active at the same time in one interpreter overwrite each other’s state, so the runner cannot be driven from several threads at once.

Warning

Do not parallelize CLI tests with threads. Use process-based isolation instead, where each worker has its own interpreter and its own copy of those globals. pytest-xdist does this with pytest --numprocesses=auto.

Click’s own test suite uses exactly this setup: the tests-random dependency group and the random tox environment in pyproject.toml run the suite under pytest-randomly and pytest-xdist to surface ordering bugs and races.

Deprecated since version 8.5.0: CliRunner.isolated_filesystem() is deprecated and will be removed in Click 9.0. Use a temporary directory with absolute paths, as shown above.

Input Streams

The test wrapper can provide input data for the input stream (stdin). This is very useful for testing prompts.

prompt.py

import click

@click.command()
@click.option('--foo', prompt=True)
def prompt(foo):
   click.echo(f"foo={foo}")

test_prompt.py

from click.testing import CliRunner
from prompt import prompt

def test_prompts():
   runner = CliRunner()
   result = runner.invoke(prompt, input='wau wau\n')
   assert not result.exception
   assert result.output == 'Foo: wau wau\nfoo=wau wau\n'

Prompts will be emulated so they write the input data to the output stream as well. If hidden input is expected then this does not happen.

Capture modes

CliRunner captures output by replacing sys.stdout and sys.stderr with in-memory wrappers. The capture parameter controls which strategy is used.

capture="sys" (default)

Captures Python-level writes (print(), click.echo(), sys.stdout.write()). It is fast and sufficient for most Click applications.

Code that holds a reference to the original sys.stdout (like a library that does from sys import stdout at import time) bypasses the capture and its output is lost.

In this mode sys.stdout.fileno() and sys.stderr.fileno() raise io.UnsupportedOperation, matching the pre-8.3.3 behavior. C-level consumers (faulthandler, subprocess, C extensions) that expect a real file descriptor must opt into the capture="fd" mode.

capture="fd"

Redirects OS file descriptors 1 and 2 to a temporary file via os.dup2(), inspired by Pytest’s capfd. This catches output that bypasses sys.stdout, including:

• Stale references to the original sys.stdout and sys.stderr.

• Logging frameworks that cache the original stream (like structlog or the stdlib’s logging module).

• C extensions and subprocesses that write directly to fd 1 or fd 2.

from click.testing import CliRunner
from myapp import cli


def test_captures_everything():
    runner = CliRunner(capture="fd")
    result = runner.invoke(cli)
    # result.stdout contains both Python-level and fd-level output
    assert "expected output" in result.stdout

In this mode sys.stdout.fileno() returns the saved (pre-redirection) fd, so faulthandler and similar consumers keep working. Writes to fd 1 and fd 2 land in the capture tmpfile, so os.dup2() calls inside the CLI no longer leak into the host runner’s stdout.

Note

capture="fd" is not available on Windows.

Changelog

Changed in version 8.4.0: Added the capture parameter. The default sys mode no longer exposes the original fd through fileno(), reverting the change introduced in 8.3.3 that broke Pytest’s fd-level capture teardown. Use capture="fd" to restore that behavior with proper isolation.