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While going developing OpenERP, I found the following piece of code
'app_date': lambda *a: time.strftime('%Y-%m-%d')
I know what lambda is.My question is why use lambda?Why not just
'app_date': time.strftime('%Y-%m-%d')
526
A lambda expression is a short block of code which takes in parameters and returns a value. Lambda expressions are similar to methods, but they do not need a name and they can be implemented right in the body of a method.
Lambda expressions basically express instances of functional interfaces (An interface with single abstract method is called functional interface. An example is java.lang.Runnable).
The "this" and "super" references within a lambda expression are the same as in the enclosing context. Since the lambda expression doesn't define a new scope, "this" keyword within a lambda expression signifies "this" parameter of a method where the lambda expression is residing.
We use lambda functions when we require a nameless function for a short period of time. In Python, we generally use it as an argument to a higher-order function (a function that takes in other functions as arguments). Lambda functions are used along with built-in functions like filter() , map() etc.
'app_date': time.strftime('%Y-%m-%d') will evaluate the time.strftime immediately. By wrapping it in a lambda, its execution is deferred until later (the time when you call the lambda). Roughly speaking, the difference is between "the time when I defined this" and "the time when I am using this". Look:
>>> d = {'a': time.time(), 'b': lambda: time.time()}
>>> d['a'], d['b']()
(1346913545.049, 1346913552.409)
>>> d['a'], d['b']()
(1346913545.049, 1346913554.518)
>>> d['a'], d['b']()
(1346913545.049, 1346913566.08)
I allowed some time to elapse in between each d['a'], d['b'](). Note that d['a'] is always the same: it is the time when I defined d. d['b'] is a function. d['b']() (with parentheses) calls the function, which evaluates the time anew on each call, so it is different at each usage.
Also, this is nothing special about lambda. Lambdas are just functions like any other. I could do the same with:
def func():
return time.time()
d = {'a': time.time(), 'b': func}
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