utilities.txt

Utilities
=========

.. currentmodule:: testfixtures

This section describes a few handy functions that didn't fit nicely in
any other section.

.. _generator:

The ``generator`` helper
------------------------

It can be handy when testing to be able to turn a simple sequence into
a generator. This can be necessary when you want to check that your
code will behave correctly when processing a generator instead of a
simple sequence, or when you're looking to make assertions about the
expected return value of a callable that returns a generator.

If you need to turn a simple sequence into a generator, the
:func:`generator` function is the way to do it:

>>> from testfixtures import generator
>>> generator(1,2,3)
<generator object ...>

.. invisible-code-block: python

  from __future__ import print_function

Iterating over this generator will return the arguments passed to the
:func:`generator` function:

>>> for i in _:
...     print(i, end=' ')
1 2 3 

The ``wrap`` helper
-------------------

The :func:`wrap` helper is a decorator function that allows you to
wrap the call to the decorated callable with calls to other
callables. This can be useful when you want to perform setup and
teardown actions either side of a test function.

For example, take the following functions:

.. code-block:: python

  def before():
      print("before")

  def after():
      print("after")

The :func:`wrap` helper can be used to wrap a function with these:

.. code-block:: python

  from testfixtures import wrap

  @wrap(before,after)
  def a_function():
      print("a_function")

When the wrapped function is executed, the output is as follows:

>>> a_function()
before
a_function
after

The section argument to :func:`wrap` is optional:

.. code-block:: python

  from testfixtures import wrap

  @wrap(before)
  def a_function():
      print("a_function")

Now, the wrapped function gives the following output when executed:

>>> a_function()
before
a_function

Multiple wrapping functions can be provided by stacking :func:`wrap`
decorations:

.. code-block:: python

  def before1():
      print("before 1")

  def after1():
      print("after 1")

  def before2():
      print("before 2")

  def after2():
      print("after 2")

  @wrap(before2,after2)
  @wrap(before1,after1)
  def a_function():
      print("a_function")

The order of execution is illustrated below:

>>> a_function()
before 1
before 2
a_function
after 2
after 1

The results of calling the wrapping functions executed before the
wrapped function can be made available to the wrapped function
provided it accepts positional arguments for these results:

.. code-block:: python

  def before1():
      return "return 1"

  def before2():
      return "return 2"

  @wrap(before2)
  @wrap(before1)
  def a_function(r1,r2):
      print(r1)
      print(r2)

Calling the wrapped function illustrates the behaviour:

>>> a_function()
return 1
return 2

Finally, the return value of the wrapped function will always be that
of the original function:

.. code-block:: python

  def before1():
      return 1

  def after1():
      return 2

  def before2():
      return 3

  def after2():
      return 4

  @wrap(before2,after2)
  @wrap(before1,after2)
  def a_function():
      return 'original'

When the above wrapped function is executed, the original return value
is still returned:

>>> a_function()
'original'