# Testing Click Applications

```{eval-rst}
.. currentmodule:: click.testing
```

Click provides the {ref}`click.testing <testing>` module to help you invoke
command line applications and check their behavior.

These tools should only be used for testing since they change
the entire interpreter state for simplicity. They are not thread-safe!

The examples use [pytest](https://docs.pytest.org/en/stable/) style tests.

```{contents}
:depth: 1
:local: true
```

## Basic Example

The key pieces are:
  - {class}`CliRunner` - used to invoke commands as command line scripts.
  - {class}`Result` - returned from {meth}`CliRunner.invoke`. Captures output
    data, exit code, optional exception, and captures the output as bytes and
    binary data.

```{code-block} python
:caption: hello.py

import click

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

```{code-block} python
:caption: 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
{meth}`CliRunner.invoke`:

```{code-block} python
:caption: 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')
```

```{code-block} python
:caption: 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 {meth}`CliRunner.invoke` will be used to
construct the initial {class}`Context object <click.Context>`.
For example, setting a fixed terminal width equal to 60:

```{code-block} python
:caption: sync.py

import click

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

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

```{code-block} python
:caption: 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

The {meth}`CliRunner.isolated_filesystem` context manager sets the current
working directory to a new, empty folder.

```{code-block} python
:caption: cat.py

import click

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

```{code-block} python
:caption: test_cat.py

from click.testing import CliRunner
from cat import cat

def test_cat():
   runner = CliRunner()
   with runner.isolated_filesystem():
      with open('hello.txt', 'w') as f:
          f.write('Hello World!')

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

Pass in a path to control where the temporary directory is created.
In this case, the directory will not be removed by Click. Its useful
to integrate with a framework like Pytest that manages temporary files.

```{code-block} python
:caption: test_cat.py

from click.testing import CliRunner
from cat import cat

def test_cat_with_path_specified():
   runner = CliRunner()
   with runner.isolated_filesystem('~/test_folder'):
      with open('hello.txt', 'w') as f:
          f.write('Hello World!')

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

## Input Streams

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

```{code-block} python
:caption: prompt.py

import click

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

```{code-block} python
:caption: 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

{class}`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
{exc}`io.UnsupportedOperation`, matching the pre-`8.3.3` behavior. C-level
consumers ({mod}`faulthandler`, {mod}`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
{func}`os.dup2`, inspired by [Pytest's
`capfd`](https://docs.pytest.org/en/stable/how-to/capture-stdout-stderr.html).
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`.

```python
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
{mod}`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.
```

```{versionchanged} 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.
```