# python-3.x Python函數定義中的 - >意思是什麼？

``````>>> def kinetic_energy(m:'in KG', v:'in M/S')->'Joules':
...    return 1/2*m*v**2
...
>>> kinetic_energy.__annotations__
{'return': 'Joules', 'v': 'in M/S', 'm': 'in KG'}
``````

``````>>> '{:,} {}'.format(kinetic_energy(20,3000),
kinetic_energy.__annotations__['return'])
'90,000,000.0 Joules'
``````

``````>>> rd={'type':float,'units':'Joules','docstring':'Given mass and velocity returns kinetic energy in Joules'}
>>> def f()->rd:
...    pass
>>> f.__annotations__['return']['type']
<class 'float'>
>>> f.__annotations__['return']['units']
'Joules'
>>> f.__annotations__['return']['docstring']
'Given mass and velocity returns kinetic energy in Joules'
``````

``````def validate(func, locals):
for var, test in func.__annotations__.items():
value = locals[var]
try:
pr=test.__name__+': '+test.__docstring__
except AttributeError:
pr=test.__name__
msg = '{}=={}; Test: {}'.format(var, value, pr)
assert test(value), msg

def between(lo, hi):
def _between(x):
return lo <= x <= hi
_between.__docstring__='must be between {} and {}'.format(lo,hi)
return _between

def f(x: between(3,10), y:lambda _y: isinstance(_y,int)):
validate(f, locals())
print(x,y)
``````

``````>>> f(2,2)
AssertionError: x==2; Test: _between: must be between 3 and 10
>>> f(3,2.1)
AssertionError: y==2.1; Test: <lambda>
``````
python python-3.x annotations function-definition

``````funcdef: 'def' NAME parameters ['->' test] ':' suite
``````

Python 2中缺少可選的“箭頭”塊，我無法在Python 3中找到任何有關其含義的信息。事實證明，這是正確的Python，它已被解釋器接受：

``````def f(x) -> 123:
return x
``````

• 我不能在這裡測試`x` ，它仍然是未定義的，
• 無論我放在箭頭後面（例如`2 < 1` ），它都不會影響功能行為。

### Tags

python   python-3.x   annotations   function-definition