We’d love you to contribute. Here are some practical hints to help out.

This document assumes you are familiar with Bash and Python.

General recommendations

  • Please, be careful with tools like autopep8, black or yapf. They may result in a massive number of changes, making pull requests harder to review. Also, when using them, use a maximum line length of 100. To avoid confusion, only clean up the code you are working on. A safer option is to use cardboardlint -F -r master. This will only clean code where you have already made changes.

  • Do not add module-level pylint: disable=... lines, except for the no-member warning in the unit test modules. When adding pylint exception, place them as locally as possible and make sure they are justified.

  • Use type hinting to document the types of function (and method) arguments and return values. This is not yet consistently done throughout IOData at the moment, but it would be helpful to do so in future pull requests. Avoid using strings to postpone the evaluation of the type. (See PEP 0563 for more details on postponed type annotation.)

  • In unit testing, use np.testing.assert_allclose and np.testing.assert_equal for comparing floating-point and integer numpy arrays respectively. np.testing.assert_allclose can also be used for comparing floating point scalars. In all other cases (not involving floating point numbers), the simple assert a == b works equally well and is more readable.

  • IOData always uses atomic units internally. See Unit conversion for details.

Adding new file formats

Each file format is implemented in a module of the package iodata.formats. These modules all follow the same API. Please consult existing formats for some guidance, e.g. the is a simple but complete example. From the following list, PATTERNS and one of the functions must be implemented:

  • PATTERNS = [ ... ]: a list of glob patterns used to recognize file formats from the file names. This is used to select the correct module from iodata.formats in functions in iodata.api.

  • load_one: load a single IOData object.

  • dump_one: dump a single IOData object.

  • load_many: load multiple IOData objects (iterator) from a single file.

  • dump_many: dump multiple IOData objects (iterator) to a single file.

load_one function: reading a single IOData object from a file

In order to read from a new file format, the module must contain a load_one function with the following signature:

@document_load_one("format", ['list', 'of', 'guaranteed', 'attributes'],
                   ['list', 'of', 'attributes', 'which', 'may', 'be', 'read'],
def load_one(lit: LineIterator) -> dict:
    """Do not edit this docstring. It will be overwritten."""
    # Actual code to read the file

The LineIterator instance provides a convenient interface for reading files and can be found in iodata.utils. As a rule of thumb, always use next(lit) to read a new line from the file. You can use this iterator in a few ways:

# When you need to read one line.
line = next(lit)

# When sections appear in a file in fixed order, you can use helper functions.
data1 = _load_helper_section1(lit)
data2 = _load_helper_section2(lit)

# When you intend to read everything in a file (not for trajectories).
for line in lit:
    # do something with line.

# When you just need to read a section.
for line in lit:
    # do something with line
    if done_with_section:

# When you need a fixed numbers of lines, say 10.
for i in range(10):
    line = next(lit)

# More complex example, in which you detect several sections and call other
# functions to parse those sections. The code is not sensitive to the
# order of the sections.
while True:
    line = next(lit)
    if end_pattern in line:
    elif line == 'section1':
        data1 = _load_helper_section1(lit)
    elif line == 'section2':
        data2 = _load_helper_section2(lit)

# Same as above, but reading till end of file. You cannot use a for loop
# when multiple lines must be read in one iteration.
while True:
        line = next(lit)
    except StopIteration:
    if end_pattern in line:
    elif line == 'section1':
        data1 = _load_helper_section1(lit)
    elif line == 'section2':
        data2 = _load_helper_section2(lit)

In some cases, one may have to push back a line because it was read too early. For example, in the Molden format, this is sometimes unavoidable. When needed you can push back the line for later reading with lit.back(line).

# When you just need to read a section
for line in lit:
    # do something with line
    if done_with_section:
        # only now it becomes clear that you've read one line to far

When you encounter a file-format error while reading the file, call lit.error(msg), where msg is a short message describing the problem. The error appearing on screen will automatically also contain the filename and line number.

dump_one functions: writing a single IOData object to a file

The dump_one functions are conceptually simpler: they just receive an open file object and an IOData instance as arguments, and should write the data to the open file.

@document_dump_one("format", ['guaranteed', 'attributes'], ['optional', 'attribtues'], notes)
def dump_one(f: TextIO, data: IOData):
    """Do not edit this docstring. It will be overwritten."""
    # code to write data to f.

load_many function: reading multiple IOData objects from a single file

This function works essentially in the same way as load_one, but can load multiple molecules. For example:

@document_load_many("XYZ", ['atcoords', 'atnums', 'title'])
def load_many(lit: LineIterator) -> Iterator[dict]:
    """Do not edit this docstring. It will be overwritten."""
    # XYZ Trajectory files are a simple concatenation of individual XYZ files,'
    # making it travial to load many frames.
    while True:
            yield load_one(lit)
        except StopIteration:

The XYZ trajectory format is simply a concatenation of individual XYZ files, such that one can use the load_one function to read a single frame. In some file formats, more complicated approaches are needed. In any case, one must use the yield keyword for every frame read from a file.

dump_many function: writing multiple IOData objects to a single file

Also dump_many is very similar to dump_one, but just takes an iterator over multiple IOData instances as argument. It is expected to write all of these to a single open file object. For example:

@document_dump_many("XYZ", ['atcoords', 'atnums'], ['title'])
def dump_many(f: TextIO, datas: Iterator[IOData]):
    """Do not edit this docstring. It will be overwritten."""
    # Similar to load_many, this is relatively easy.
    for data in datas:
        dump_one(f, data)

Also here, we take advantage of the simple structure of the XYZ trajectory format, i.e. the simple concatenation of individual XYZ files. For other formats, this could become more complicated.

Github work flow

Before diving into technicalities: if you intend to make major changes, beyond fixing bugs and small functionality improvements, please open a Github issue first, so we can discuss before coding. Please explain what you intend to accomplish and why. That often saves a lot of time and trouble in the long run.

Use the issue to plan your changes. Try to solve only one problem at a time, instead of fixing several issues and adding different features in a single shot. Small changes are easier to handle, also for the reviewer in the last step below.

Mention in the corresponding issue when you are working on it. “Claim” the issue to avoid duplicate efforts.

  1. Check your GitHub settings and your local git configuration:

    • If you don’t have an SSH key pair yet, create one with the following terminal command:

      ssh-keygen -t rsa -b 4096 -C ""

      A suitable name for this key would be id_rsa_github. An empty pass phrase is convenient and should be fine. This will generate a private and a public key in ${HOME}/.ssh.

    • Upload your public SSH key to This is a single long line in, which you can copy-paste into the browser.

    • Configure SSH to use this key pair for authentication when pushing branches to Github. Add the following to your .ssh/config file:

          ForwardX11 no
          IdentityFile /home/your_user_name/.ssh/id_rsa_github

      (Make sure you have the correct path to the private key file.)

    • Configure git to use the name and e-mail address tied to your Github account:

      git config --global "Your Name"
      git config --global ""
  2. Install Roberto, which is the driver for our CI setup. It can also replicate the continuous integration on your local machine, which makes it easier to prepare a passable pull request. See

  3. Make a fork of the project, using the Github “fork” feature.

  4. Clone the original repository on your local machine and enter the directory

    git clone
    cd iodata
  5. Add your fork as a second remote to your local repository, for which we will use the short name mine below, but any short name is fine:

    git remote add mine<your-github-account>/iodata.git
  6. Make a new branch, with a name that hints at the purpose of your modification:

    git checkout -b new-feature
  7. Make changes to the source. Please, make it easy for others to understand your code. Also, add tests that verify your code works as intended. Rules of thumb:

    • Write transparent code, e.g. self-explaining variable names.

    • Add comments to passages that are not easy to understand at first glance.

    • Write docstrings explaining the API.

    • Add unit tests when feasible.

  8. Commit your changes with a meaningful commit message. The first line is a short summary, written in the imperative mood. Optionally, this can be followed by an empty line and a longer description.

    If you feel the summary line is too short to describe what you did, it may be better to split your changes into multiple commits.

  9. Run Roberto and fix all problems it reports. Either one of the following should work

    rob                 # Normal case
    python3 -m roberto  # Only if your PATH is not set correctly

    Style issues, failing tests and packaging issues should all be detected at this stage.

  10. Push your branch to your forked repository on Github:

    git push mine -u new-feature

    A link should be printed on screen, which will take the next step for you.

  11. Make a pull request from your branch new-feature in your forked repository to the master branch in the original repository.

  12. Wait for the tests on Travis-CI to complete. These should pass. Also coverage analysis will be shown, but this is merely indicative. Normally, someone should review your pull request in a few days. Ideally, the review results in minor corrections at worst. We’ll do our best to avoid larger problems in step 1.

Notes on attrs

IOData uses the attrs library, not to be confused with the attr library, for classes representing data loaded from files: IOData, MolecularBasis, Shell, MolecularOrbitals and Cube. This enables basic attribute validation, which eliminates potentially silly bugs. (See iodata/ and the usage of validate_shape in all those classes.)

The following attrs functions could be convenient when working with these classes:

  • The data can be turned into plain Python data types with the attr.asdict function. Make sure you add the retain_collection_types=True option, to avoid the following issue: For example.

    from iodata import load_one
    import attr
    iodata = load_one("")
    fields = attr.asdict(iodata, retain_collection_types=True)

    A similar astuple function works as you would expect.

  • A shallow copy with a few modified attributes can be created with the evolve method, which is a wrapper for attr.evolve:

    from iodata import load_one
    import attr
    iodata1 = load_one("")
    iodata2 = attr.evolve(iodata1, title="another title")

    The usage of evolve becomes mandatory when you want to change two or more attributes whose shape need to be consistent. For example, the following would fail:

    from iodata import IOData
    iodata = IOData(atnums=[7, 7], atcoords=[[0, 0, 0], [2, 0, 0]])
    # The next line will fail because the size of atnums and atcoords
    # becomes inconsistent.
    iodata.atnums = [8, 8, 8]
    iodata.atcoords = [[0, 0, 0], [2, 0, 1], [4, 0, 0]]

    The following code, which has the same intent, does work:

       from iodata import IOData
       import attr
       iodata1 = IOData(atnums=[7, 7], atcoords=[[0, 0, 0], [2, 0, 0]])
       iodata2 = attr.evolve(
           atnums=[8, 8, 8],
           atcoords=[[0, 0, 0], [2, 0, 1], [4, 0, 0]],
    For brevity, lists (of lists) were used in these examples. These are always
    converted to arrays by the constructor or when assigning them to attributes.