.. _axes-handling:

Axes handling
*************

.. _dimensions-label:

The navigation and signal dimensions
------------------------------------

HyperSpy distinguishes between *signal* and *navigation* axes and most
functions operate on the *signal* axes and iterate over the *navigation* axes.
Take an EELS spectrum image as specific example. It is a 2D array of spectra
and has three dimensions: X, Y and energy-loss. In HyperSpy, X and Y are the
*navigation* dimensions and the energy-loss is the *signal* dimension. To make
this distinction more explicit, the representation of the object includes a
separator ``|`` between the navigation and signal dimensions. In analogy, the
*signal* dimension in EDX would be the X-ray energy, in optical spectra the
wavelength axis, etc. However, HyperSpy can also handle data with more than one
*signal* dimension, such as a stack or even map of diffraction images or
electron-holograms in TEM.

For example: A spectrum image has signal dimension 1 and navigation dimension 2
and is stored in the Signal1D subclass.

.. code-block:: python

    >>> s = hs.signals.Signal1D(np.zeros((10, 20, 30)))
    >>> s
    <Signal1D, title: , dimensions: (20, 10|30)>


An image stack has signal dimension 2 and navigation dimension 1 and is stored
in the Signal2D subclass.

.. code-block:: python

    >>> im = hs.signals.Signal2D(np.zeros((30, 10, 20)))
    >>> im
    <Signal2D, title: , dimensions: (30|20, 10)>


A map of images has signal dimension 2 and navigation dimension 2 and is stored
in the Signal2D subclass.

.. code-block:: python

    >>> im = hs.signals.Signal2D(np.zeros((30, 10, 10, 20)))
    >>> im
    <Signal2D, title: , dimensions: (10, 30|20, 10)>


.. _Setting_axis_properties:

Setting axis properties
-----------------------

The axes are managed and stored by the :class:`~.axes.AxesManager` class
that is stored in the :attr:`~.api.signals.BaseSignal.axes_manager` attribute of
the signal class. The individual axes can be accessed by indexing the
:class:`~.axes.AxesManager`, e.g.:

.. code-block:: python

    >>> s = hs.signals.Signal1D(np.random.random((10, 20 , 100)))
    >>> s
    <Signal1D, title: , dimensions: (20, 10|100)>
    >>> s.axes_manager
    <Axes manager, axes: (20, 10|100)>
                Name |   size |  index |  offset |   scale |  units
    ================ | ====== | ====== | ======= | ======= | ======
         <undefined> |     20 |      0 |       0 |       1 | <undefined>
         <undefined> |     10 |      0 |       0 |       1 | <undefined>
    ---------------- | ------ | ------ | ------- | ------- | ------
         <undefined> |    100 |      0 |       0 |       1 | <undefined>
    >>> s.axes_manager[0]
    <Unnamed 0th axis, size: 20, index: 0>

The navigation axes come first, followed by the signal axes. Alternatively,
it is possible to selectively access the navigation or signal dimensions:

.. code-block:: python

    >>> s.axes_manager.navigation_axes[1]
    <Unnamed 1st axis, size: 10, index: 0>
    >>> s.axes_manager.signal_axes[0]
    <Unnamed 2nd axis, size: 100>

For the given example of two navigation and one signal dimensions, all the
following commands will access the same axis:

.. code-block:: python

    >>> s.axes_manager[2]
    <Unnamed 2nd axis, size: 100>
    >>> s.axes_manager[-1]
    <Unnamed 2nd axis, size: 100>
    >>> s.axes_manager.signal_axes[0]
    <Unnamed 2nd axis, size: 100>

The axis properties can be set by setting the :class:`~.axes.BaseDataAxis`
attributes, e.g.:

.. code-block:: python

    >>> s.axes_manager[0].name = "X"
    >>> s.axes_manager[0]
    <X axis, size: 20, index: 0>


Once the name of an axis has been defined it is possible to request it by its
name e.g.:

.. code-block:: python

    >>> s.axes_manager["X"]
    <X axis, size: 20, index: 0>
    >>> s.axes_manager["X"].scale = 0.2
    >>> s.axes_manager["X"].units = "nm"
    >>> s.axes_manager["X"].offset = 100


It is also possible to set the axes properties using a GUI by calling the
:meth:`~.axes.AxesManager.gui` method of the :class:`~.axes.AxesManager`

.. code-block:: python

    >>> s.axes_manager.gui() # doctest: +SKIP

.. _axes_manager_gui_image:

.. figure::  images/axes_manager_gui_ipywidgets.png
   :align:   center

   AxesManager ipywidgets GUI.

or, for a specific axis, the respective method of e.g.
:class:`~.axes.UniformDataAxis`:

.. code-block:: python

    >>> s.axes_manager["X"].gui() # doctest: +SKIP

.. _data_axis_gui_image:

.. figure::  images/data_axis_gui_ipywidgets.png
   :align:   center

   UniformDataAxis ipywidgets GUI.

To simply change the "current position" (i.e. the indices of the navigation
axes) you could use the navigation sliders:

.. code-block:: python

    >>> s.axes_manager.gui_navigation_sliders() # doctest: +SKIP

.. _navigation_sliders_image:

.. figure::  images/axes_manager_navigation_sliders_ipywidgets.png
   :align:   center

   Navigation sliders ipywidgets GUI.

Alternatively, the "current position" can be changed programmatically by
directly accessing the ``indices`` attribute of a signal's
:class:`~.axes.AxesManager` or the ``index`` attribute of an individual
axis. This is particularly useful when trying to set
a specific location at which to initialize a model's parameters to
sensible values before performing a fit over an entire spectrum image. The
``indices`` must be provided as a tuple, with the same length as the number of
navigation dimensions:

.. code-block:: python

    >>> s.axes_manager.indices = (5, 4)

.. _Axes_properties:

Summary of axis properties
^^^^^^^^^^^^^^^^^^^^^^^^^^

* ``name`` (str) and ``units`` (str) are basic parameters describing an axis
  used in plotting. The latter enables the :ref:`conversion of units
  <quantity_and_converting_units>`.
* ``navigate`` (bool) determines, whether it is a navigation axis.
* ``size`` (int) gives the number of elements in an axis.
* ``index`` (int) determines the "current position for a navigation axis and
  ``value`` (float) returns the value at this position.
* ``low_index`` (int) and ``high_index`` (int) are the first and last index.
* ``low_value`` (int) and ``high_value`` (int) are the smallest and largest
  value.
* The ``axis`` array stores the values of the axis points. However,
  depending on the type of axis, this array may be updated from the **defining
  attributes** as discussed in the following section.


.. _Axes_types:

Types of data axes
------------------

HyperSpy supports different *data axis types*, which differ in how the axis is
defined:

* :class:`~.axes.DataAxis` defined by an array ``axis``,
* :class:`~.axes.FunctionalDataAxis` defined by a function ``expression`` or
* :class:`~.axes.UniformDataAxis` defined by the initial value ``offset``
  and spacing ``scale``.

The main disambiguation is whether the
axis is **uniform**, where the data points are equidistantly spaced, or
**non-uniform**, where the spacing may vary. The latter can become important
when, e.g., a spectrum recorded over a *wavelength* axis is converted to a
*wavenumber* or *energy* scale, where the conversion is based on a ``1/x``
dependence so that the axis spacing of the new axis varies along the length
of the axis. Whether an axis is uniform or not can be queried through the
property ``is_uniform`` (bool) of the axis.

Every axis of a signal object may be of a different type. For example, it is
common that the *navigation* axes would be *uniform*, while the *signal* axes
are *non-uniform*.

When an axis is created, the type is automatically determined by the attributes
passed to the generator. The three different axis types are summarized in the
following table.

.. table:: BaseDataAxis subclasses.

    +----------------------------------------+------------------------+----------------+
    |  BaseDataAxis subclass                 |  Defining attributes   | ``is_uniform`` |
    +========================================+========================+================+
    |  :class:`~.axes.DataAxis`              |         axis           |  False         |
    +----------------------------------------+------------------------+----------------+
    |  :class:`~.axes.FunctionalDataAxis`    |      expression        |  False         |
    +----------------------------------------+------------------------+----------------+
    |  :class:`~.axes.UniformDataAxis`       |    offset, scale       |  True          |
    +----------------------------------------+------------------------+----------------+

.. NOTE::

    Not all features are implemented for non-uniform axes.


.. warning::

    Non-uniform axes are in beta state and its API may change in a minor release.
    Not all hyperspy features are compatible with non-uniform axes and support
    will be added in future releases.


.. _uniform-data-axis:

Uniform data axis
^^^^^^^^^^^^^^^^^

The most common case is the :class:`~.axes.UniformDataAxis`. Here, the axis
is defined by the ``offset``, ``scale`` and ``size`` parameters, which determine
the `initial value`, `spacing` and `length`, respectively. The actual ``axis``
array is automatically calculated from these three values. The ``UniformDataAxis``
is a special case of the ``FunctionalDataAxis`` defined by the function
``scale * x + offset``.

Sample dictionary for a :class:`~.axes.UniformDataAxis`:

.. code-block:: python

    >>> dict0 = {'offset': 300, 'scale': 1, 'size': 500}
    >>> s = hs.signals.Signal1D(np.ones(500), axes=[dict0])
    >>> s.axes_manager[0].get_axis_dictionary()
    {'_type': 'UniformDataAxis', 'name': None, 'units': None, 'navigate': False, 'is_binned': False, 'size': 500, 'scale': 1.0, 'offset': 300.0}

Corresponding output of :class:`~.axes.AxesManager`:

.. code-block:: python

    >>> s.axes_manager
    <Axes manager, axes: (|500)>
                Name |   size |  index |  offset |   scale |  units 
    ================ | ====== | ====== | ======= | ======= | ====== 
    ---------------- | ------ | ------ | ------- | ------- | ------ 
         <undefined> |    500 |      0 |   3e+02 |       1 | <undefined> 


.. _functional-data-axis:

Functional data axis
^^^^^^^^^^^^^^^^^^^^

Alternatively, a :class:`~.axes.FunctionalDataAxis` is defined based on an
``expression`` that is evaluated to yield the axis points. The `expression`
is a function defined as a ``string`` using the
`SymPy <https://docs.sympy.org/latest/tutorial/intro.html>`_ text expression
format. An example would be ``expression = a / x + b``. Any variables in the
expression, in this case ``a`` and ``b`` must be defined as additional
attributes of the axis. The property ``is_uniform`` is automatically set to
``False``.

``x`` itself is an instance of :class:`~.axes.BaseDataAxis`. By default,
it will be a :class:`~.axes.UniformDataAxis` with ``offset = 0`` and
``scale = 1`` of the given ``size``. However, it can also be initialized with
custom ``offset`` and ``scale`` values. Alternatively, it can be a non
uniform :class:`~.axes.DataAxis`.

Sample dictionary for a :class:`~.axes.FunctionalDataAxis`:

.. code-block:: python

    >>> dict0 = {'expression': 'a / (x + 1) + b', 'a': 100, 'b': 10, 'size': 500}
    >>> s = hs.signals.Signal1D(np.ones(500), axes=[dict0])
    >>> s.axes_manager[0].get_axis_dictionary()
    {'_type': 'FunctionalDataAxis', 'name': None, 'units': None, 'navigate': False, 'is_binned': False, 'expression': 'a / (x + 1) + b', 'size': 500, 'x': {'_type': 'UniformDataAxis', 'name': None, 'units': None, 'navigate': False, 'is_binned': False, 'size': 500, 'scale': 1.0, 'offset': 0.0}, 'a': 100, 'b': 10}

Corresponding output of :class:`~.axes.AxesManager`:

.. code-block:: python

    >>> s.axes_manager
    <Axes manager, axes: (|500)>
                Name |   size |  index |  offset |   scale |  units 
    ================ | ====== | ====== | ======= | ======= | ====== 
    ---------------- | ------ | ------ | ------- | ------- | ------ 
         <undefined> |    500 |      0 | non-uniform axis | <undefined> 


Initializing ``x`` with ``offset`` and ``scale``:

.. code-block:: python

    >>> from hyperspy.axes import UniformDataAxis
    >>> dict0 = {'expression': 'a / x + b', 'a': 100, 'b': 10, 'x': UniformDataAxis(size=10,offset=10,scale=0.1)}
    >>> s = hs.signals.Signal1D(np.ones(500), axes=[dict0])
    >>> # the x array
    >>> s.axes_manager[0].x.axis
    array([10. , 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9])
    >>> # the actual axis array
    >>> s.axes_manager[0].axis
    array([20.        , 19.9009901 , 19.80392157, 19.70873786, 19.61538462,
       19.52380952, 19.43396226, 19.34579439, 19.25925926, 19.17431193])


Initializing ``x`` as non-uniform :class:`~.axes.DataAxis`:

.. code-block:: python

    >>> from hyperspy.axes import DataAxis
    >>> dict0 = {'expression': 'a / x + b', 'a': 100, 'b': 10, 'x': DataAxis(axis=np.arange(1,10)**2)}
    >>> s = hs.signals.Signal1D(np.ones(500), axes=[dict0])
    >>> # the x array
    >>> s.axes_manager[0].x.axis
    array([ 1,  4,  9, 16, 25, 36, 49, 64, 81])
    >>> # the actual axis array
    >>> s.axes_manager[0].axis
    array([110.        ,  35.        ,  21.11111111,  16.25      ,
            14.        ,  12.77777778,  12.04081633,  11.5625    ,
            11.2345679 ])

Initializing ``x`` with ``offset`` and ``scale``:


.. _data-axis:

(non-uniform) Data axis
^^^^^^^^^^^^^^^^^^^^^^^

A :class:`~.axes.DataAxis` is the most flexible type of axis. The axis
points are directly given by an array named ``axis``. As this can be any
array, the property ``is_uniform`` is automatically set to ``False``.


Sample dictionary for a :class:`~.axes.DataAxis`:

.. code-block:: python

    >>> dict0 = {'axis': np.arange(12)**2}
    >>> s = hs.signals.Signal1D(np.ones(12), axes=[dict0])
    >>> s.axes_manager[0].get_axis_dictionary()
    {'_type': 'DataAxis', 'name': None, 'units': None, 'navigate': False, 'is_binned': False, 'axis': array([  0,   1,   4,   9,  16,  25,  36,  49,  64,  81, 100, 121])}

Corresponding output of :class:`~.axes.AxesManager`:

.. code-block:: python

    >>> s.axes_manager
    <Axes manager, axes: (|12)>
                Name |   size |  index |  offset |   scale |  units 
    ================ | ====== | ====== | ======= | ======= | ====== 
    ---------------- | ------ | ------ | ------- | ------- | ------ 
         <undefined> |     12 |      0 | non-uniform axis | <undefined> 


.. _defining-axes:

Defining a new axis
-------------------

An axis object can be created through the ``axes.create_axis()`` method, which
automatically determines the type of axis by the given attributes:

.. code-block:: python

    >>> from hyperspy import axes
    >>> axis = axes.create_axis(offset=10,scale=0.5,size=20)
    >>> axis
    <Unnamed axis, size: 20>

Alternatively, the creator of the different types of axes can be called
directly:

.. code-block:: python

    >>> from hyperspy import axes
    >>> axis = axes.UniformDataAxis(offset=10,scale=0.5,size=20)
    >>> axis
    <Unnamed axis, size: 20>

The dictionary defining the axis is returned by the ``get_axis_dictionary()``
method:

.. code-block:: python

    >>> axis.get_axis_dictionary()
    {'_type': 'UniformDataAxis', 'name': None, 'units': None, 'navigate': False, 'is_binned': False, 'size': 20, 'scale': 0.5, 'offset': 10.0}

This dictionary can be used, for example, in the :ref:`initilization of a new
signal<signal_initialization>`.


Adding/Removing axes to/from a signal
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Usually, the axes are directly added to a signal during :ref:`signal
initialization<signal_initialization>`. However, you may wish to add/remove
axes from the :class:`~.axes.AxesManager` of a signal.

Note that there is currently no consistency check whether a signal object has
the right number of axes of the right dimensions. Most functions will however
fail if you pass a signal object where the axes do not match the data
dimensions and shape.

You can *add a set of axes* to the :class:`~.axes.AxesManager` by passing either a list of
axes dictionaries to ``axes_manager.create_axes()``:

.. code-block:: python

    >>> dict0 = {'offset': 300, 'scale': 1, 'size': 500}
    >>> dict1 = {'axis': np.arange(12)**2}
    >>> s.axes_manager.create_axes([dict0,dict1])

or a list of axes objects:

.. code-block:: python

    >>> from hyperspy.axes import UniformDataAxis, DataAxis
    >>> axis0 = UniformDataAxis(offset=300,scale=1,size=500)
    >>> axis1 = DataAxis(axis=np.arange(12)**2)
    >>> s.axes_manager.create_axes([axis0,axis1])

*Remove an axis* from the :class:`~.axes.AxesManager` using ``remove()``, e.g. for the last axis:

.. code-block:: python

    >>> s.axes_manager.remove(-1)


.. _quantity_and_converting_units:

Using quantity and converting units
-----------------------------------

The ``scale`` and the ``offset`` of each :class:`~.axes.UniformDataAxis` axis
can be set and retrieved as quantity.

.. code-block:: python

    >>> s = hs.signals.Signal1D(np.arange(10))
    >>> s.axes_manager[0].scale_as_quantity
    <Quantity(1.0, 'dimensionless')>
    >>> s.axes_manager[0].scale_as_quantity = '2.5 µm'
    >>> s.axes_manager
    <Axes manager, axes: (|10)>
                Name |   size |  index |  offset |   scale |  units 
    ================ | ====== | ====== | ======= | ======= | ====== 
    ---------------- | ------ | ------ | ------- | ------- | ------ 
         <undefined> |     10 |      0 |       0 |     2.5 |     µm 
    >>> s.axes_manager[0].offset_as_quantity = '2.5 nm'


Internally, HyperSpy uses the `pint <https://pint.readthedocs.io>`_ library to
manage the scale and offset quantities. The ``scale_as_quantity`` and
``offset_as_quantity`` attributes return pint object:

.. code-block:: python

    >>> q = s.axes_manager[0].offset_as_quantity
    >>> type(q) # q is a pint quantity object
    <class 'pint.Quantity'>
    >>> q
    <Quantity(2.5, 'nanometer')>


The ``convert_units`` method of the :class:`~.axes.AxesManager` converts
units, which by default (no parameters provided) converts all axis units to an
optimal unit to avoid using too large or small numbers.

Each axis can also be converted individually using the ``convert_to_units``
method of the :class:`~.axes.UniformDataAxis`:

.. code-block:: python

    >>> axis = hs.hyperspy.axes.UniformDataAxis(size=10, scale=0.1, offset=10, units='mm')
    >>> axis.scale_as_quantity
    <Quantity(0.1, 'millimeter')>
    >>> axis.convert_to_units('µm')
    >>> axis.scale_as_quantity
    <Quantity(100.0, 'micrometer')>


.. _Axes_storage_ordering:

Axes storage and ordering
-------------------------

Note that HyperSpy rearranges the axes when compared to the array order. The
following few paragraphs explain how and why.

Depending on how the array is arranged, some axes are faster to iterate than
others. Consider an example of a book as the dataset in question. It is
trivially simple to look at letters in a line, and then lines down the page,
and finally pages in the whole book.  However, if your words are written
vertically, it can be inconvenient to read top-down (the lines are still
horizontal, it's just the meaning that's vertical!). It is very time-consuming
if every letter is on a different page, and for every word you have to turn 5-6
pages. Exactly the same idea applies here - in order to iterate through the
data (most often for plotting, but for any other operation as well), you
want to keep it ordered for "fast access".

In Python (more explicitly `numpy`), the "fast axes order" is `C order` (also
called row-major order). This means that the **last** axis of a numpy array is
fastest to iterate over (i.e. the lines in the book). An alternative ordering
convention is `F order` (column-major), where it is the other way round: the
first axis of an array is the fastest to iterate over. In both cases, the
further an axis is from the `fast axis` the slower it is to iterate over this
axis. In the book analogy, you could think about reading the first lines of
all pages, then the second and so on.

When data is acquired sequentially, it is usually stored in acquisition order.
When a dataset is loaded, HyperSpy generally stores it in memory in the same
order, which is good for the computer. However, HyperSpy will reorder and
classify the axes to make it easier for humans. Let's imagine a single numpy
array that contains pictures of a scene acquired with different exposure times
on different days. In numpy, the array dimensions are  ``(D, E, Y, X)``. This
order makes it fast to iterate over the images in the order in which they were
acquired. From a human point of view, this dataset is just a collection of
images, so HyperSpy first classifies the image axes (``X`` and ``Y``) as
`signal axes` and the remaining axes the `navigation axes`. Then it reverses
the order of each set of axes because many humans are used to get the ``X``
axis first and, more generally, the axes in acquisition order from left to
right. So, the same axes in HyperSpy are displayed like this: ``(E, D | X,
Y)``.

Extending this to arbitrary dimensions, by default, we reverse the numpy axes,
chop them into two chunks (signal and navigation), and then swap those chunks,
at least when printing. As an example:

.. code-block:: bash

    (a1, a2, a3, a4, a5, a6) # original (numpy)
    (a6, a5, a4, a3, a2, a1) # reverse
    (a6, a5) (a4, a3, a2, a1) # chop
    (a4, a3, a2, a1) (a6, a5) # swap (HyperSpy)

In the background, HyperSpy also takes care of storing the data in memory in
a "machine-friendly" way, so that iterating over the navigation axes is always
fast.

.. _iterating_axesmanager:

Iterating over the AxesManager
------------------------------
One can iterate over the :class:`~.axes.AxesManager` to produce indices to
the navigation axes. Each iteration will yield a new tuple of indices, sorted
according to the iteration path specified in :attr:`~.axes.AxesManager.iterpath`.
Setting the :attr:`~.axes.AxesManager.indices` property to a new index will
update the accompanying signal so that signal methods that operate at a specific
navigation index will now use that index, like ``s.plot()``.

.. code-block:: python

    >>> s = hs.signals.Signal1D(np.zeros((2,3,10)))
    >>> s.axes_manager.iterpath # check current iteration path
    'serpentine'
    >>> for index in s.axes_manager:
    ...     print(index)
    (0, 0)
    (1, 0)
    (2, 0)
    (2, 1)
    (1, 1)
    (0, 1)

The :attr:`~.axes.AxesManager.iterpath` attribute specifies the strategy that
the :class:`~.axes.AxesManager` should use to iterate over the navigation axes.
Two built-in strategies exist:

- ``'serpentine'`` (default): starts at (0, 0), but when it reaches the final column
  (of index N), it continues from (1, N) along the next row, in the same way
  that a snake might slither, left and right.
- ``'flyback'``: starts at (0, 0), continues down the row until the final
  column, "flies back" to the first column, and continues from (1, 0).



.. code-block:: python

    >>> s = hs.signals.Signal1D(np.zeros((2,3,10)))
    >>> s.axes_manager.iterpath = 'flyback'
    >>> for index in s.axes_manager:
    ...     print(index)
    (0, 0)
    (1, 0)
    (2, 0)
    (0, 1)
    (1, 1)
    (2, 1)


The :attr:`~.axes.AxesManager.iterpath` can also be set using the
:meth:`~.axes.AxesManager.switch_iterpath` context manager:

.. code-block:: python

    >>> s = hs.signals.Signal1D(np.zeros((2,3,10)))
    >>> with s.axes_manager.switch_iterpath('flyback'):
    ...     for index in s.axes_manager:
    ...         print(index)
    (0, 0)
    (1, 0)
    (2, 0)
    (0, 1)
    (1, 1)
    (2, 1)

The :attr:`~.axes.AxesManager.iterpath` can also be set to be a specific list of indices, like [(0,0), (0,1)],
but can also be any generator of indices. Storing a high-dimensional set of
indices as a list or array can take a significant amount of memory. By using a
generator instead, one almost entirely removes such a memory footprint:

.. code-block:: python

    >>> s.axes_manager.iterpath = [(0,1), (1,1), (0,1)]
    >>> for index in s.axes_manager:
    ...     print(index)
    (0, 1)
    (1, 1)
    (0, 1)

    >>> def reverse_flyback_generator():
    ...     for i in reversed(range(3)):
    ...         for j in reversed(range(2)):
    ...             yield (i,j)

    >>> s.axes_manager.iterpath = reverse_flyback_generator()
    >>> for index in s.axes_manager:
    ...     print(index)    
    (2, 1)
    (2, 0)
    (1, 1)
    (1, 0)
    (0, 1)
    (0, 0)


Since generators do not have a defined length, and does not need to include all
navigation indices, a progressbar will be unable to determine how long it needs
to be. To resolve this, a helper class can be imported that takes both a generator
and a manually specified length as inputs:

.. code-block:: python

    >>> from hyperspy.axes import GeneratorLen
    >>> gen = GeneratorLen(reverse_flyback_generator(), 6)
    >>> s.axes_manager.iterpath = gen