Developer Guide

The primary use case for Flatten Tool is to convert spreadsheets to JSON so that the data can be validated using a JSON Schema.

Flatten Tool has to be very forgiving in what it accepts so that it can deal with spreadsheets that are a work-in-progress. It tries its best to make sense of what you give it, even if you give it inconsistent, conflicting or patchy data. It leaves the work of reporting problems to the JSON Schema validator that will be run on the JSON it produces, and it only generates warnings if it is forced to ignore data from the source spreadsheet.

Flatten Tool tries its best to output as much as it can so the JSON it produces will be as good or bad as the spreadsheet input it receives. The benefit of this approach that the user can be shown all the problems in one go when the JSON Schema validator is run on that JSON.

Programming a very forgiving tool that tries to accept lots of categories of errors is a lot more complex than programming a tool where the data structures are very predictable. Understanding this intention not to raise errors is key to understanding Flatten Tool’s internal design.

Helper libraries

As you’ll have read in the User Guide, Flatten Tool makes use of JSON Pointer, JSON Schema and JSON Ref standards. The Python libraries that support this are jsonpointer, jsonschema and jsonref respectively.

Running the tests

After following the installation above, run py.test.

Note that the tests require the Python test suite. This should come with Python, but some distributions split it out. On Ubuntu you will need to install a package like libpython3.5-testsuite (depending on which Python version you are using).

Testing coverage of documentation examples

rm -f .coverage # Remove the old coverage if it exists
python flattentool/tests/
coverage combine
coverage report --omit=flattentool/tests/**

Versioning and CHANGELOG

This library is versioned and a changelog is kept in the file. See that file for more.

Any pull request to this library should include updating the file.


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What’s coming up

Three layer design

The codebase will be refactored so that the unflatten part of the library comes in three parts:

Spreadsheet Loaders

Responsible for loading data out of spreadsheets and representing it in the correct format for the unflattener - a Python structure of basic JSON types and the special Empty value

Unflatten function

Takes the Python data structure described above and unflattens it, using a JSON Schema if present and keeping all state explicit.

Use the JSON Schema to convert any basic JSON types to richer types that can be correctly serialised by a serialiser later (e.g. dates). Returns a cell tree.


Take a look at the run() function in flattentool/tests/ to see a function that behaves a little like a pure Python entry point to Flatten Tool’s functionality.


Take a cell tree and serialise it to either a JSON tree, a source map, or both

This pattern will make it easier to support testing the core unflatten function, as well as making it easier to support future spreadsheet and serialiser formats.

Explicit float support

The existing implementation makes a special effort to correctly handle decimal types such as currency.

This special effort also means that Flatten Tool treats float values as Decimal too.

Most of the time this is perfectly fine, since Python correctly treats a Decimal generated from a float as being equal to the float itself:

>>> from decimal import Decimal
>>> Decimal(1.3) == 1.3

Do be aware of this small quirk of Python’s behaviour though. Python doesn’t treat a Decimal obtained from '1.3' as being the same as one generated from 1.3:

>>> Decimal('1.3') == Decimal(1.3)
>>> Decimal(1.3)

Stdin support

The next version could support a single sheet being fed into stdin like this:

cat << EOF | flatten-tool unflatten -f=csv --root-list-path=cafe
Healthy Cafe,

More documentation

  • Flattening, roll up and template creation
  • Timezone support
  • Using Flatten Tool as a library
  • Source maps


We could name the command line tool flattentool rather than flatten-tool so that everything is consistent.