Directives

A cheat sheet of directives available in nbdev.

Directives are special comments that are preceded by #| that control:

1. Cell visibility in rendered documentation
2. How source code is generated from notebook cells
3. Execution of cells for tests and docs

nbdev augments Quarto by providing additional directives than what are available in Quarto. All Quarto directives can be used in nbdev notebooks.

This cheat sheet lists all nbdev directives in addition to some Quarto directives we believe are important. We recommend consulting the quarto docs to see all of the directives available to you.

To clarify the origin of directives we use the following emojis:

• 📓 for nbdev-only directives .
• 🔵 for Quarto directives (which also work in nbdev).

Cell Visibility

The following directives control cell visibility in rendered documentation:

📓 #|hide

Hide cell input and output.

Example

The following will result in the contents of the cell and it’s output from being hidden:

#|hide
print('you will not see this')

Note that using #|hide is equivalent to using the Quarto directive #|include: false:

#|include: false
print('you will not see this')

See the quarto docs for more information about #|include.

🔵 #|echo: <true|false>

Toggle the visibility of code-cell inputs.

Example

#|echo: false
print('you can see the output but not the code!')

which results in:

you can see the output but not the code!

🔵 #|output: <true|false|asis>

Setting this to false hides the output of a cell. Setting this to asis renders the output as raw markdown.

Example

The following cell will not display any output:

#|output: false
1 + 1

The following cell with #|output: asis will produce the output hello fastai rendered as markdown instead of a string:

#|output: asis
print("`hello fastai`")

📓 #|hide_line

Hide a specific line of code in an input cell.

Example

def _secret(): ...

for i in range(3):
    _secret() #|hide_line
    print(i)

becomes this:

def _secret(): ...

for i in range(3):
    print(i)

0
1
2

📓 #|filter_stream <keyword> ...

Filter lines containing specific keywords in cell outputs.

Example

#|filter_stream FutureWarning MultiIndex
print('\n'.join(['A line', 'Foobar baz FutureWarning blah', 
                 'zig zagMultiIndex zoom', 'Another line.']))

will output this:

A line
Another line.

🔵 #|code-fold: <show|true>

The #|code-fold directive allows you to collapse code cells. When set to true, the element is collapsed by default, when set to show show the element is shown by default.

Example

When you set #|code-fold: true, the input cell is collapsed:

print('this is')
print('output')
print('that takes')
print('lots of vertical space')

this is
output
that takes
lots of vertical space

When you set #|code-fold: show the input cell is shown but still in a collapsible element:

print('this is')
print('output')
print('that takes')
print('lots of vertical space')

this is
output
that takes
lots of vertical space

Generating Source Code

The following directives control how source code is exported from code cells.

📓 #|default_exp <name>

Names the module where cells with the #|export directive will be exported to by default.

Example

#| default_exp baz

# In a new notebook cell:

#| export
def my_function(): pass

If our package is named: bitsnbytes then we can do:

from bitsnbytes.baz import my_function

You can define the package name by using lib_name in settings.ini.

📓 #|export

Exports the items in the cell into the generated module and documentation.

Example

#|export
def say_hello(to:str # name of person to say hello to
             ):
    "Say hello to somebody"
    return f'Hello {to}!'

The above cell will get exported to the module specified by #|default_exp. These exports are automatically included in __all__ for the module. To learn how export without inclusion in __all__, see the #|exporti directive.

Furthermore, the documentation for this function will automatically be rendered like this:

---

say_hello

 say_hello (to:str)

Say hello to somebody

	Type	Details
to	str	name of person to say hello to

The docs are generated from this export using show_doc. See these docs for a detailed discussion of show_doc.

📓 #|exporti

An internal export. Not included in __all__ or the docs. Useful for a function that is called by other functions in this module but is not part of the public API.

Equivalently, you can prefix your function or method with _ e.g. def _private(): pass.

📓 #|exports

A source export. Like #|export but in addition to showing docs via showdoc.show_doc, it also shows the source code.

Example

#|exports
def say_hello(to):
    "Say hello to somebody"
    return f'Hello {to}!'

this will produce the following output:

def say_hello(to):
    "Say hello to somebody"
    return f'Hello {to}!'

---

say_hello

 say_hello (to)

Say hello to somebody

Cell Execution

The following directives allow you to control how cells are executed during docs rendering and testing.

📓 #|exec_doc

Ensures that a cell is executed each time before generating docs. When a cell does not have this annotation, it is run according to the default rules described here.

Example

datetime.datetime.now()

datetime.datetime(2022, 8, 18, 9, 1, 43, 907609)

However with the annotation:

#|exec_doc
datetime.datetime.now()

we can see that the time has been updated:

datetime.datetime.now()

datetime.datetime(2024, 12, 5, 19, 32, 41, 655838)

🔵 #|eval: <true|false>

When set to false, the cell is ignored during testing.

Example

#|eval: false
raise Exception("I'm not raised because I'm not run")

Cell execution when there is no directive

When a cell has no directives, cells are run by nbdev according to the behavior described here.