Contexts

Context

The context defines how a reference (f.A, f['A'], f.A.B) is evaluated

Context.EVAL

This context evaluates references directly. For example, if data is {"a": 1}, f["a"] evaluates to 1.

from pipda import register_verb, Context

@register_verb(dict, context=Context.EVAL)
def mutate(data, **kwargs):
    data = data.copy()
    data.update(kwargs)
    return data

{"a": 1} >> mutate(b=f["a"] * 2)  # {"a": 1, "b": 2}

Context.SELECT

This context evaluates references into the attribute names or subscripts themselves. For example, f.key works as a shortcut for "key".

@register_verb(dict, context=Context.SELECT)
def select(data, *keys):
    return {key: val for key, val in data.items() if key in keys}

{"a": 1, "b": 2} >> select(f.a)  # {"a": 1}

Context.PENDING

This keeps expressions unevaluated so they are passed into the function and can be evaluated later inside the function.

from pipda import Context, register_verb, evaluate_expr

@register_verb(dict, context=Context.PENDING)
def mutate(data, **kwargs):
    # kwargs is holding expressions
    kwargs = evaluate_expr(kwargs, data, Context.EVAL)
    data = data.copy()
    data.update(kwargs)
    return data

{"a": 1} >> mutate(b=f["a"] * 2)  # {"a": 1, "b": 2}

Customizing a context

You can write your own context by subclassing ContextBase. Overwrite the following methods to define the behaviors:

• getattr: How to evaluate f.A
• getitem: How to evaluate f["A"]
• ref (property): How to evaluate x in f[x]

from pipda import Context, ContextBase, register_verb

class MyContext(ContextBase):
    def getattr(self, parent, ref, level):
        # f.A -> level 1
        # f.A.B -> level 2
        return parent[ref]

    def getitem(self, parent, ref, level):
        return ref

@register_verb(dict, context=MyContext())
def subset_and_update(data, *cols, **kwargs):
    data = {key: val for key, val in data.items() if key in cols}
    data.update(kwargs)
    return data

{"a": 1, "b": 2, "c": 3} >> subset_and_update(f.a, f.b, a=f.c)
# {"a": 3, "b": 2}

Note

For Context.PENDING, subclass context.ContextPending to keep expressions unevaluated.

Contexts for keyword arguments

We can set extra contexts for keyword arguments.

@register_verb(
    dict,
    context=Context.EVAL,
    kw_context={"cols": Context.SELECT},
)
def subset_and_update(data, *, cols, **kwargs):
    data = {key: val for key, val in data.items() if key in cols}
    data.update(kwargs)
    return data

{"a": 1, "b": 2, "c": 3} >> subset_and_update(f.a, f.b, a=f.c)
# {"a": 3, "b": 2}

Note

By default, the context is None. Expressions — including FunctionCall and VerbCall objects — will await the next available context for evaluation.

Note

When registering additional types for a verb, contexts and extra contexts are inherited from the first type registered with register_verb.

Tip

Each implementation for the registered verbs or functions can have its own contexts. Specify them when registering the implementations.

<verb>.register(..., context=..., kw_context=...)
<func>.register(..., context=..., kw_context=...)