Lazy evaluation in Python

Question

What is lazy evaluation in Python?

One website said :

In Python 3.x the range() function returns a special range object which computes elements

Answer 1

A github repo named python patterns and wikipedia tell us what lazy evaluation is.

Delays the eval of an expr until its value is needed and avoids repeated evals.

range in python3 is not a complete lazy evaluation, because it doesn't avoid repeated eval.

A more classic example for lazy evaluation is cached_property:

import functools

class cached_property(object):
    def __init__(self, function):
        self.function = function
        functools.update_wrapper(self, function)

    def __get__(self, obj, type_):
        if obj is None:
            return self
        val = self.function(obj)
        obj.__dict__[self.function.__name__] = val
        return val

The cached_property(a.k.a lazy_property) is a decorator which convert a func into a lazy evaluation property. The first time property accessed, the func is called to get result and then the value is used the next time you access the property.

eg:

class LogHandler:
    def __init__(self, file_path):
        self.file_path = file_path

    @cached_property
    def load_log_file(self):
        with open(self.file_path) as f:
            # the file is to big that I have to cost 2s to read all file
            return f.read()

log_handler = LogHandler('./sys.log')
# only the first time call will cost 2s.
print(log_handler.load_log_file)
# return value is cached to the log_handler obj.
print(log_handler.load_log_file)

To use a proper word, a python generator object like range are more like designed through call_by_need pattern, rather than lazy evaluation