Reading Code – Assertions & Assumptions

I'm fascinated by how engineers read and interpret code that they work on a daily basis. It's no doubt different for everyone, but few explain how they go about and understand a unfamiliar codebase.

For me, being comfortable in a codebase usually means two things:

  • Being able to jump to relevant, related parts of the code.
  • Understand the implications of changing a piece of code
    • Inside the system – impact of code quality. How does the code influence code in the same project?
    • Outside the system – implications for human and program collaborators. How does the code influence other projects, teams, or users?

Both can be addressed by following the data flow of the code. I refer to my strategy as assertions and assumptions. Take this arbitrary code snippet from Django:

# from django/forms/
def fields_for_model(model, fields=None, exclude=None, widgets=None,
                     formfield_callback=None, localized_fields=None,
                     labels=None, help_texts=None, error_messages=None,
    field_list = []
    ignored = []
    opts = model._meta
    # Avoid circular import
    from django.db.models.fields import Field as ModelField
    sortable_private_fields = [f for f in opts.private_fields if isinstance(f, ModelField)]
    for f in sorted(chain(opts.concrete_fields, sortable_private_fields, opts.many_to_many)):
        if not getattr(f, 'editable', False):
            if (fields is not None and in fields and
                    (exclude is None or not in exclude)):
                raise FieldError(
                    "'%s' cannot be specified for %s model form as it is a non-editable field" % (
              , model.__name__)
        if fields is not None and not in fields:
        if exclude and in exclude:

        kwargs = {}
        if widgets and in widgets:
            kwargs['widget'] = widgets[]
        if localized_fields == ALL_FIELDS or (localized_fields and in localized_fields):
            kwargs['localize'] = True
        if labels and in labels:
            kwargs['label'] = labels[]
        if help_texts and in help_texts:
            kwargs['help_text'] = help_texts[]
        if error_messages and in error_messages:
            kwargs['error_messages'] = error_messages[]
        if field_classes and in field_classes:
            kwargs['form_class'] = field_classes[]

        if formfield_callback is None:
            formfield = f.formfield(**kwargs)
        elif not callable(formfield_callback):
            raise TypeError('formfield_callback must be a function or callable')
            formfield = formfield_callback(f, **kwargs)

        if formfield:
            field_list.append((, formfield))
    field_dict = OrderedDict(field_list)
    if fields:
        field_dict = OrderedDict(
            [(f, field_dict.get(f)) for f in fields
                if ((not exclude) or (exclude and f not in exclude)) and (f not in ignored)]
    return field_dict

Without looking much into the other code around it, we can make (roughly) two kinds of interferences: assumptions and assertions in alternating fashion.

1. Assumptions

  • This code probably has to deal with html formdata handling based on the name and project (django being a web framework)
  • It seems to be a public method because the function's name isn't prefixed with an underscore for private / protected functions as per python's coding conventions
    • There's also no other private or protected indicator that may indicate that if the developer was for accustomed to programming in another language.
  • The function name indicates getting some fields (a collection?) given a model (a form model?)

2. Assertions

  • This code loops over fields specified in the Model's _meta field.
  • This function returns an OrderedDict type.
    • The keys are the field's name property and the values are something called a formfield.
  • Form fields are constructed with kwargs map that the loop spends most of its time building.
  • Form fields are sortable.
  • There is a special case handled for Django's database models.
  • widgets, localized_fields, labels, help_texts, error_messages, field_classes is dictionary-like with field names as keys.
  • formfield_callback is a callable (aka - function)
  • fields and exclude are list-like
  • is hashable and support equality

3. Assumptions

  • FormFields must be some class. They may be different from regular Field classes, or they could be related via inheritance or abstract base-class. If this code wasn't python, then could also be an interface.
  • fields and exclude are probably whitelists and blacklists respectively
  • formfield_callback is an alternative constructor for these formfield instances
  • It looks like this function read's some or all of a models' fields. Models are from the database or elsewhere. The return value is a dictionary of field names to some field model.

This cycle of assumptions and assertions can be repeated as many times as needed. Exploring more code that is related will help convert some assumptions into assertions (eg - jumping to a piece of code that uses this function). But the goal is to build a collection of assertions you can make about the code at any given point in time.

Navigating through the Code

Navigating through code is primarily driven by the need to convert assumptions into assertions. In the example above, I happened to pick a piece of code at random. But usually exploring a codebase is driven by a goal – usually to make a change to the software.

When I navigate down into more implementation details, the goal is to understand how or what is being accomplish in a piece of code. This means I need to understand how a piece of code works, what arguments a function or class expects, etc.

Learning context of the a given code's execution when I navigate up to find usages of the code I was reading. This helps understand the bigger picture of why the some code was written and if it can make assumptions about its execution. It's only natural for a function to be implemented against the cases it's known to encounter.


In short:

  • Don't trust names of things. The human language is a highly context-specific form of communication. Original meanings are easily lost to time. Never trust names of things unless you have verified it yourself.
  • Code is always contextual to decisions both inside and outside the codebase. Since time changes the context, code must also change over time.
  • Even if you don't use a strongly-typed language, it is still valuable to think in terms of how a type checker works.