# 5 Best Ways to Return a Scalar Type Common to Input Arrays in Python

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π‘ Problem Formulation: In Python, when handling multiple arrays, often there is a need to determine a data type that is common across all of themβa scalar type to which all elements can be safely cast without losing information. For example, if you have input arrays [1, 2] and [3.0, 4.5], a common scalar type would be float, as integers can be cast to floats without loss of precision.

## Method 1: Using numpy.find_common_type()

NumPy provides a function `numpy.find_common_type()` which examines arrays to find a common data type that can be safely used for all arrays. It analyzes the input arrays’ data types and returns the minimum common type that ensures no loss of information when values are cast to the resulting type.

Here’s an example:

```import numpy as np

arrays = [np.array([1, 2]), np.array([3.0, 4.5])]
common_type = np.find_common_type([arr.dtype for arr in arrays], [])
print(common_type)```

Output:

`float64`

By calling `numpy.find_common_type()` with a list of data types extracted from the arrays’ `dtype` attribute, the function harmonizes the types to a common one, here determining that `float64` is suitable for both input arrays.

## Method 2: Using reduction with type promotion

An alternative approach without NumPy is to manually implement a reduce-like function that iterates through the types of all array elements, and uses Python’s built-in type promotion logic to determine the common type. This is a more hands-on method, relying on Python’s type coercion rules.

Here’s an example:

```from functools import reduce

def find_common_type(types):
return reduce(lambda x, y: type(np.promote_types(np.dtype(x), np.dtype(y)).type()), types)

arrays = [[1, 2], [3.0, 4.5]]
types = {type(item) for sublist in arrays for item in sublist}
common_type = find_common_type(types)
print(common_type)```

Output:

`<class 'numpy.float64'>`

The code employs a custom function `find_common_type()` that combines all element types using `reduce()` and `np.promote_types()`. This promotes types to a common standard, similar to NumPy’s internal mechanisms, yielding `numpy.float64` as the scalar type.

## Method 3: Checking Types with isinstance()

This technique involves iterating through all elements of the input arrays and utilizing the `isinstance()` function to check against a predefined hierarchy of types, from more precise (like integers) to more generic ones (like complex numbers).

Here’s an example:

```arrays = [[1, 2], [3.0, 4.5]]
type_hierarchy = (int, float, complex)

def find_common_type(arrays, type_hierarchy):
common_type = type_hierarchy[0]
for array in arrays:
for item in array:
for _type in type_hierarchy:
if isinstance(item, _type):
common_type = _type
break
return common_type

common_type = find_common_type(arrays, type_hierarchy)
print(common_type)```

Output:

`<class 'float'>`

The function `find_common_type()` uses `isinstance()` to test each element against the type hierarchy until it finds a match, then updates the common type if necessary. This process finds the least generic data type that all elements can be cast to, which in this case is `float`.

## Method 4: Union of Type Sets

In this method, Python’s set types are used to create a union of the types from all input arrays. By establishing a sorting order in the union of the types set, we can determine the most common type that all elements can be converted to without loss of information.

Here’s an example:

```arrays = [[1, 2], [3.0, 4.5]]
type_order = {int: 0, float: 1, complex: 2}

sets_of_types = (set(map(type, sublist)) for sublist in arrays)
common_types = set.union(*sets_of_types)
common_type = sorted(common_types, key=lambda x: type_order[x])[0]
print(common_type)```

Output:

`<class 'float'>`

The script creates sets of types for each array, then finds the union of these sets. By defining a type hierarchy in a dictionary `type_order`, it sorts the aggregated types to get the lowest common denominator. The result is the `float` class, which is the most suitable common type for the given arrays’ elements.

## Bonus One-Liner Method 5: Using max() with a Custom Key

A concise one-liner can be crafted using the `max()` function with a custom key function that maps each type to its rank in the type hierarchy. Itβs a quick and elegant way to find the common scalar type for small and simple arrays.

Here’s an example:

```arrays = [[1, 2], [3.0, '4.5']]
common_type = max({type(item) for sublist in arrays for item in sublist}, key=({int: 0, float: 1, complex: 2, str: 3}).get)
print(common_type)```

Output:

`<class 'str'>`

With a single line, all types are collected into a set using a set comprehension, then `max()` identifies the ‘greatest’ type based on the hierarchy defined in the key function, resulting in the identification of `str` as the common scalar type in this particular example.

## Summary/Discussion

• Method 1: Using numpy.find_common_type(). It’s reliable and leverages NumPy’s optimized algorithms, but requires numpy as a dependency.
• Method 2: Reduction with type promotion. It’s more adaptable to custom logic, but is less concise and requires an understanding of Python’s coercion rules.
• Method 3: Checking Types with isinstance(). Straightforward for simple type hierarchies, though it may become complex with more varied data types.
• Method 4: Union of Type Sets. Uses basic Python set operations and is fairly intuitive, but sorting by custom criteria could become cumbersome.
• Bonus Method 5: Using max() with a custom key. Short and elegant, although less readable and potentially error-prone with complicated hierarchies.