Write snippets of code in your documentation and execute them as regression tests.

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byexample supports Python.

Run the Python examples calling byexample as:

$ byexample -l python your-file-here                # byexample: +skip
$ byexample -l python3 your-file-here               # byexample: +skip

I’m assuming that you have Python installed in your system as byexample needs it to run but just in case, this is the download page for this interpreter.

Stability: stable - non backward compatibility changes, if happen, they will between major versions.

You can enable Python with -l python and since byexample 10.0.2 the explicit -l python3 is supported too.

If -l python is used in the command line, byexample will use the binary named python while if you use -l python3 it will be the binary named python3.

Versions tested

We tested byexample with the following versions of the language and the underlying runner or interpreter:

Language Runner/Interpreter
3.6 3.6.15
3.7 3.7.13
3.8 3.8.12
3.9 3.9.12
3.10 3.10.4

Find interactive examples

For Python, byexample uses the >>> string as the primary and ... as the secondary prompts.

>>> def f():
...    return 42

>>> f()

Python 2.x support

byexample 8.x.x and below has full support for running examples written in Python 2.x as well as in Python 3.x.

However the January 1st of 2020 is the end of life of Python 2.7 and byexample 9 will drop support for it.

Compatibility with doctest

In fact, byexample is inspired by the Python’s doctest module.

I borrowed a few ideas from it and I also tried to overcome its issues.

This makes a Python example to look very similar to a doctest but it is not fully compatible with it.

To make it (almost) fully compatible, you need to pass the +py-doctest flag to byexample in the command line.

In the following examples I will pass this flag in the examples themselves.

If you enabled the compatibility from the command line, nothing needs to be changed: you can still using the doctest keyword to change the options of the example.

But because I’m enabling it from the example itself, I need use the byexample keyword instead of doctest

>>> [1, 2, 3]   # use byexample, not doctest --> # byexample: +py-doctest  +NORMALIZE_WHITESPACE
[1,   2,   3]

As you can see NORMALIZE_WHITESPACE is supported.


>>> True   # byexample: +py-doctest +SKIP

>>> raise Exception("foo")   # byexample: +py-doctest +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
 -- stack trace
 -- ignored
module.ignored.Exception: -- this will be ignored --

>>> print("foo\n<BLANKLINE>\nbar")   # byexample: +py-doctest +DONT_ACCEPT_BLANKLINE

>>> print("foo\n\nbar")   # byexample: +py-doctest

And also the report flags: REPORT_UDIFF, REPORT_CDIFF and REPORT_NDIFF

As you may guess, the byexample’s capture tags feature are disabled in this compatibility mode.

But in the other hand, you can use the ELLIPSIS flag as usual.

>>> print("fooxxxbar")   # byexample: +py-doctest +ELLIPSIS

Compatibility overview

============================= ============================
``doctest``                   Observations
============================= ============================
``DONT_ACCEPT_TRUE_FOR_1``    Ignored
``ELLIPSIS``                  Supported
``SKIP``                      Supported
``REPORT_UDIFF``              Supported
``REPORT_CDIFF``              Supported
``REPORT_NDIFF``              Supported
``FAIL_FAST``                 Supported
============================= ============================

DONT_ACCEPT_TRUE_FOR_1 is not supported as it was implemented in doctest as a workaround for the result of a comparison in Python 2.3: in that time Python returned 1 and 0 instead of True and False.


There is not distinction between a normal output and an exception so if one want to ignore the traceback, one need to use <...>

This is different from doctest where the exceptions are captured and handled different from other outputs. This enables doctest to know when an exception was raised but in the practice is not critical.

If you didn’t enabled the compatibility with doctest, the <...> is enabled by default.

>>> raise Exception('oh no!')
Traceback <...>
Exception: oh no!

>>> non_existent_var
Traceback <...>
NameError: name 'non_existent_var' is not defined

At difference with doctest, syntax errors are also captured.

>>> f(]        # invalid syntax
SyntaxError: <...>

If you enabled the compatibility mode, any output that it looks like an exception will be captured and mangled like doctest does: the traceback header and the stacktrace are ignored.

>>> raise Exception('oh no!')       # byexample: +py-doctest
Traceback (most recent call last):
 -- stack trace
 -- ignored
Exception: oh no!

Also note that we will relax the check of the prefix of the exception message.

In Python 2.x it was common to print only the exception class name but in Python 3.x, the default is to print the full exception name (a dotted name including its modules)

Testing this it is hard in doctest.

One option is ignore the exception’s details but this also disables the check of the whole exception message defeating the purpose of checking an exception.

The other was to use ... but doctest doesn’t allow to use it at the begin of a line.

Because of this, add a <...> at the begin of the message to avoid these quirks.

As a side effect, we can check loosely the name of an exception:

>>> raise ValueError('oh no!')       # byexample: +py-doctest
Traceback (most recent call last):
Error: oh no!

>>> raise IndexError('oh no!')       # byexample: +py-doctest
Traceback (most recent call last):
Error: oh no!

Migration to the byexample’s way

As you can see byexample uses a different set of options. Here is a summary of the equivalent options:

====================  ============================  ============================
``byexample``         ``doctest``                   Observations
====================  ============================  ============================
``norm-ws``           ``NORMALIZE_WHITESPACE``      Same functionality.
*not supported*       ``DONT_ACCEPT_TRUE_FOR_1``    Only useful for ``Python 2.3``.
``tags``              ``ELLIPSIS``                  More powerful than ``doctest`` version
``skip``              ``SKIP``                      Same functionality.
``pass``              *not supported*               Execute but do not check.
``tags``              ``IGNORE_EXCEPTION_DETAIL``   ``tags`` is more general
``tags`` or ``rm``    ``DONT_ACCEPT_BLANKLINE``     ``rm`` may be used instead of ``tags``.
``diff``              ``REPORT_UDIFF``              With ``unified`` as argument.
``diff``              ``REPORT_CDIFF``              With ``context`` as argument.
``diff``              ``REPORT_NDIFF``              With ``ndiff`` as argument.
``fail-fast``         ``FAIL_FAST``                 Same functionality.
``show-failures`      ``REPORT_ONLY_FIRST_FAILURE`` Same as `+show-failures 1`
====================  ============================= ============================

DONT_ACCEPT_BLANKLINE and IGNORE_EXCEPTION_DETAIL are used to ignore some pieces of the output. The tags flag of byexample should cover those cases and even more.

See norm-ws, tags, skip, pass and diff for more info.

Pretty print

By default, byexample uses a custom display hook based on the Python’s pprint module.

The custom display hook will pretty print the object instead of using repr

>>> l = ["foo bar %i" % i for i in range(10)]
>>> l
['foo bar 0',
 'foo bar 1',
 'foo bar 2',
 'foo bar 3',
 'foo bar 4',
 'foo bar 5',
 'foo bar 6',
 'foo bar 7',
 'foo bar 8',
 'foo bar 9']

If we don’t do this, long and complex structures could be hard to print:

>>> print(repr(l))
['foo bar 0', 'foo bar 1', <...>, 'foo bar 8', 'foo bar 9']

This feature is disabled if you enabled the doctest compatibility mode but it can be reenabled with +py-pretty-print.

Note that byexample uses the pprint module of the Python interpreter running the example. pprint doesn’t warranty that its output will be stable between Python versions. Keep that in mind.

In the future, byexample may provide a different pprint stable implementation.

Bytes/Unicode marker (byexample 8.1.3 or below)

Python 2.x uses u' and u" (and U' and U") to mark the begin of an unicode literal. Optionally one can use b' to mark the begin of a sequence of bytes (str in Python 2.x)

Unfortunately, in Python 3.x it is the u' marker optional and the b' marker mandatory.

This duality forces to have two different sets of expected results one for Python 2.x and other for Python 3.x or do not relay in the pprint functionality for testing at all plus some dirty hacks.

In byexample 8.1.3 or below, the Python interpreter of byexample used a custom pretty printer to remove all the markers u' and b' for simple and for nested objects retaining the original alignment.

This changed in byexample 9.0.0 and the markers of the bytes are left unchanged.

Changed in byexample 9.0.0: the bytes will have their usual b' marker.

The following are valid examples for Python 3.x but not in 2.x

>>> u = u'foo'
>>> b = b'bar'

>>> u

>>> b

>>> du = {u'aaaaaaaa': {u'bbbbbbbbbb': u'asasaaaaaaaaaaaaaasasa', u'c': u'asaaaaaaaaaaaaaaaaaaaaa'}}
>>> db = {b'aaaaaaaa': {b'bbbbbbbbbb': b'asasaaaaaaaaaaaaaasasa', b'c': b'asaaaaaaaaaaaaaaaaaaaaa'}}

>>> du
{'aaaaaaaa': {'bbbbbbbbbb': 'asasaaaaaaaaaaaaaasasa',
              'c': 'asaaaaaaaaaaaaaaaaaaaaa'}}

>>> db
{b'aaaaaaaa': {b'bbbbbbbbbb': b'asasaaaaaaaaaaaaaasasa',
               b'c': b'asaaaaaaaaaaaaaaaaaaaaa'}}

>>> b'b'

>>> u'u'

The pretty print is disabled if you are in compatibility mode with doctest. If you find it useful but you cannot leave the compatibility mode, you can set the +py-pretty-print flag to enable it.


Custom prompt

Internally, we change the primary and secondary prompts to a non trivial texts to reduce the probability of a collision with the code to be executed and with the output returned by the interpreter.

>>> ">>> "
'>>> '

>>> "... "
'... '

>>> sys
Traceback <...>
NameError: name 'sys' is not defined

Empty lines

Consider the following function definition. It is obvious for a human being that the return statement belongs to the function definition.

>>> def foo():
...   a = 42
...   return a

But Python interprets the empty line between a = 42 and return a as the end of the definition.

byexample removes any empty line that it is followed by an indented line so the whole example makes sense to Python.

>>> i = 0
>>> for j in range(2):
...   i += j
... print(i)

Keep in mind that “empty line” means that, if it is not working for you double check for any trailing whitespace.

We can disable this fix with -py-remove-empty-lines

>>> def foo():      # byexample: -py-remove-empty-lines
...   a = 42
...   return a
  File <...>
    return a
<...>IndentationError: unexpected indent

You may ask why if the byexample fix works, why anyone would like to disable it. Well, the fix comes with some side effects.

See the following multiline string definition

>>> blob = '''
...   foo
... '''

How many lines it has? 4 right? well….

>>> blob
'\n  foo\n'

>>> len(blob.split('\n'))

It has actually 4 but byexample suppress the empty line because it is followed by a indented line foo so we got 3.

In my personal experience, I didn’t find an issue with this in the field but if you need to disable it, you can

>>> # byexample: -py-remove-empty-lines
... blob = '''
...   foo
... '''

>>> blob
'\n\n  foo\n'

>>> len(blob.split('\n'))

Known limitations (byexample 8.1.3 or below)

Python 3 has a healthier handling of unicode and bytes than Python 2 and it always warranties a valid output given an encoding (sys.stdout.encoding), typically utf-8.

But in Python 2 there is a mix. If you try to print arbitrary bytes, Python 2 will not complain even if the resulting output is not a valid utf-8 output.

byexample and other third parties tools cannot distinguish the arbitrary bytes from the correct ones. In those cases, the example will fail.

>>> print(b"\x00\x1ffoo\xff\xff")                       # byexample: +skip

Changed in byexample 9.0.0: Python 2.x is not longer supported and the limitation was removed.

Python specific options

$ byexample -l python --show-options       # byexample: +norm-ws
python's specific options
  +py-doctest           enable the compatibility with doctest.
  +py-pretty-print      enable the pretty print enhancement.
                        enable the deletion of empty lines (enabled by
                        [doctest] alias for +norm-ws.
  +SKIP                 [doctest] alias for +skip.
  +ELLIPSIS             [doctest] enables the ... wildcard.
                        [doctest] take <BLANKLINE> as literal.
                        [doctest] ignored.
                        [doctest] ignore the exception details.
  +REPORT_UDIFF         [doctest] alias for +diff unified.
  +REPORT_CDIFF         [doctest] alias for +diff context.
  +REPORT_NDIFF         [doctest] alias for +diff ndiff.
  +FAIL_FAST            [doctest] alias for +fail-fast.
                        [doctest] alias for +show-failures 1.