Continuum Analytics: Documentation

# Interactive Expressions

Internally Blaze is abstract; this limits interactivity. Blaze interactive expressions resolve this issue and provide a smooth experience to handling foreign data.

## Expressions with Data

Internally Blaze separates the intent of the computation from the data/backend. While powerful, this abstract separation limits interactivity, one of the core goals of Blaze.

Blaze interactive expressions are like normal expressions but their leaves may hold on to a concrete data resource (like a DataFrame or SQL database.) This embeds a specific data context, providing user interface improvements at the cost of full generality.

## Example

We create an interactive expression by calling the `data` constructor on any object or URI with which Blaze is familiar.

    >>> from blaze import data, Symbol
    >>> from blaze.utils import example
    >>> db = data('sqlite:///%s' % example('iris.db'))  # an interactive expression
    >>> db.iris
        sepal_length  sepal_width  petal_length  petal_width      species
    0            5.1          3.5           1.4          0.2  Iris-setosa
    1            4.9          3.0           1.4          0.2  Iris-setosa
    2            4.7          3.2           1.3          0.2  Iris-setosa
    3            4.6          3.1           1.5          0.2  Iris-setosa
    4            5.0          3.6           1.4          0.2  Iris-setosa
    5            5.4          3.9           1.7          0.4  Iris-setosa
    6            4.6          3.4           1.4          0.3  Iris-setosa
    7            5.0          3.4           1.5          0.2  Iris-setosa
    8            4.4          2.9           1.4          0.2  Iris-setosa
    9            4.9          3.1           1.5          0.1  Iris-setosa
    ...

    >>> db.iris.species.<tab>
    db.iris.species.columns       db.iris.species.max
    db.iris.species.count         db.iris.species.min
    db.iris.species.count_values  db.iris.species.ndim
    db.iris.species.distinct      db.iris.species.nunique
    db.iris.species.dshape        db.iris.species.relabel
    db.iris.species.expr          db.iris.species.resources
    db.iris.species.fields        db.iris.species.schema
    db.iris.species.head          db.iris.species.shape
    db.iris.species.isidentical   db.iris.species.sort
    db.iris.species.label         db.iris.species.species
    db.iris.species.like          db.iris.species.to_html
    db.iris.species.map

    >>> db.iris.species.distinct()
               species
    0      Iris-setosa
    1  Iris-versicolor
    2   Iris-virginica

In the case above `db` is a `Symbol`, just like any normal Blaze leaf expresion

    >>> isinstance(db, Symbol)
    True

But `db` has one additional field, `db.data` which points to a SQLAlchemy Table.

    >>> db.data
    <sqlalchemy.Table at 0x7f0f64ffbdd0>

Compute calls including `db` may omit the customary namespace, e.g.

    >>> from blaze import compute
    >>> expr = db.iris.species.distinct()

    >>> # compute(expr, {db: some_sql_object})  # Usually provide a namespace
    >>> compute(expr)
    ['Iris-setosa', 'Iris-versicolor', 'Iris-virginica']

This implicit namespace can be found with the `._resources` method

    >>> expr._resources()
    {db: <sqlalchemy.Table object>}

Additionally, we override the `__repr__` and `_repr_html_` methods to include calls to `compute`. This way, whenever an expression is printed to the screen a small computation is done to print the computed data instead.

As an example, this `__repr__` function looks something like the following:

    from odo import odo
    from pandas import DataFrame
    from blaze import Expr

    def __repr__(expr):
        expr = expr.head(10)         # Only need enough to print to the screen
        result = compute(expr)       # Do the work necessary to get a result
        df = odo(result, DataFrame) # Shove into a DataFrame
        return repr(df)              # Use pandas' nice printing

    Expr.__repr__ = __repr__   # Override normal __repr__ method

This provides smooth interactive feel of interactive expressions. Work is only done when an expression is printed to the screen and excessive results are avoided by wrapping all computations in a `.head(10)`.