Handpick

Handpick is a tool to work with nested data structures.

Installation

pip install handpick

Quick introduction

The pick function

The pick generator function performs the recursive traversal of a nested data structure and picks all objects that meet certain criteria provided in the form of a predicate function. Picked objects are retrieved lazily by an iterator.

Simple predicate functions

The predicate function is passed to pick as the predicate argument. For example:

from handpick import pick

def is_non_empty_string(obj):
    return isinstance(obj, str) and obj

data = [[1, ""], [-2, ["foo", 3.0]], -4, "bar"]

>>> for s in pick(data, predicate=is_non_empty_string):
...     print(s)
...
foo
bar

Handling dictionary keys

When traversing mappings like dictionaries, you can configure whether or not pick will examine dictionary keys by specifying the dict_keys keyword argument. Default is False, which means only dictionary values are examined. For example:

from handpick import pick

data = {"foo": {"name": "foo"}, "bar": {"name": "bar"}}

>>> for s in pick(data, predicate=lambda obj: "a" in obj):
...     print(s)
...
bar
>>> for s in pick(data, predicate=lambda obj: "a" in obj, dict_keys=True):
...     print(s)
...
name
bar
name
bar

Predicates

The Predicate decorator

The Predicate decorator wraps a function in an object that can be combined with other predicates using the operators & (and) and | (or), as well as negated using the operator ~ (not).

Combining predicates

For example:

from handpick import pick, Predicate

@Predicate
def is_integer(obj):
    return isinstance(obj, int)

@Predicate
def is_even(number):
    return number % 2 == 0

data = [[4, [5.0, 1], 3.0], [[15, []], {17: [7, [8], 0]}]]

# compound predicate
odd_int = is_integer & ~is_even

>>> for n in pick(data, predicate=odd_int):
...     print(n)
...
1
15
7

Combining predicates with functions

In addition, the & and | operations are supported between predicates and regular undecorated functions. For example:

from handpick import pick, Predicate

@Predicate
def is_list(obj):
    return isinstance(obj, list)

data = [("1", [2]), {("x",): [(3, [4]), "5"]}, ["x", ["6"]], {7: ("x",)}]

# compound predicate
short_list = (lambda obj: len(obj) < 2) & is_list

>>> for l in pick(data, predicate=short_list):
...     print(l)
...
[2]
[4]
['6']

Suppressing errors

The important thing to note is that when the predicate's underlying function raises an exception, the exception is suppressed and the predicate returns False. In other words, it is assumed that the object in question does not meet the picking criteria. For example:

from handpick import pick, Predicate

@Predicate
def above_zero(number):
    return number > 0

>>> above_zero(1)
True
>>> above_zero("a")
False
>>> for n in pick([[1, "Py", -2], [None, 3.0]], predicate=above_zero):
...     print(n)
...
1
3.0

In the example above, several lists and strings were internally compared to 0 but no TypeError propagated up to the code that called above_zero.

Predicate factories

The is_type function can be used to create predicates based on an object's type. For example:

from handpick import pick, is_type

data = [[1.0, [2, True]], [False, [3]], ["4"]]

strictly_int = is_type(int) & ~is_type(bool)

>>> for n in pick(data, predicate=strictly_int):
...     print(n)
...
2
3

The no_error function can be used to create predicates based on whether a function applied to an object raises an error.

from handpick import pick, is_type, no_error

data = {"name": "spam", "price": "15.42", "quantity": 68, "year": "2011"}

# strings that can be cast to floats
numeric_str = is_type(str) & no_error(float)

>>> for s in pick(data, predicate=numeric_str):
...     print(s)
...
15.42
2011

Useful functions

The values_for_key function

When inspecting data structures that contain dictionaries or other mappings, you can use values_for_key to retrieve values associated with a specific key, regardless of the nested depth in which these values are stored. Values are retrieved lazily by an iterator. For example:

from handpick import values_for_key

data = {
    "node_id": 4,
    "child_nodes": [
        {
            "node_id": 8,
            "child_nodes": [
                {
                    "node_id": 16,
                },
            ],
        },
        {
            "id": 9,
        },
    ],
}

>>> for i in values_for_key(data, key="node_id"):
...     print(i)
...
4
8
16

Multiple keys may be specified at a time. For example:

>>> for i in values_for_key(data, key=["node_id", "id"]):
...     print(i)
...
4
8
16
9

The max_depth function

This function returns the maximum nested depth of a data structure. For example:

>>> from handpick import max_depth
>>> max_depth([0, [1, [2]]])
2
>>> max_depth({0: {1: {2: {3: {4: 4}}}}})
4

Note: Just like non-empty collections, empty collections constitute another level of nested depth. For example:

>>> max_depth([0, [1, []]])
2

Recipes

Flattening nested data

To flatten a list of lists, use the pick function without the predicate argument and pass collections=False. For example:

from handpick import pick

data = [[], [0], [[[], 1], [2, [3, [4]], []], [5]]]

>>> list(pick(data, collections=False))
[0, 1, 2, 3, 4, 5]

API reference

pick

handpick.pick(data, predicate=None, *, collections=True, dict_keys=False, bytes_like=False)

Pick objects from data based on predicate.

Traverse data recursively and yield all objects for which predicate(obj) is True or truthy. data should be an iterable collection.

predicate must be callable, must take one argument, and should return a Boolean value. If predicate is omitted or None, all objects are picked.

By default, collections of other objects are yielded just like any other objects. To exclude collections, pass collections=False.

When traversing a mapping, only its values are inspected by default. To inspect both keys and values of mappings, pass dict_keys=True.

By default, bytes-like sequences (bytes and bytearrays) are not treated as collections of other objects and therefore not iterated by the recursive algorithm. This can be changed by passing bytes_like=True.

Strings are not treated as collections of other objects and therefore not iterated by the recursive algorithm.

Predicate

@handpick.Predicate(func=None, *, suppressed_errors=(TypeError, ValueError, LookupError, AttributeError))

Decorator wrapping a function in a predicate object.

The decorated function can be combined with other predicates using the operators & (and) and | (or), as well as negated using the operator ~ (not).

suppressed_errors can be used to customize which exception classes will be suppressed by the predicate.

Predicate objects are intended to be used as the predicate argument to the pick function.

is_type

handpick.is_type(type_or_types)

Predicate factory. Return a predicate that returns True if object is an instance of specified type(s).

type_or_types must be a type or tuple of types.

no_error

handpick.no_error(func)

Predicate factory. Return a predicate that returns True if func can be applied on object without an exception being raised, False otherwise.

values_for_key

handpick.values_for_key(data, key)

Pick values associated with a specific key.

Traverse data recursively and yield a sequence of dictionary values that are mapped to key. key may be a list of multiple keys.

max_depth

handpick.max_depth(data)

Return maximum nested depth of data.

data should be an iterable collection. Depth is counted from zero, i.e. the direct elements of data are in depth 0.