Question

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')

Answer 1

'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}