最近在查看Python 3.3语法规范时，我注意到一些有趣的事情:
funcdef: 'def' NAME parameters ['->' test] ':' suite
可选的“箭头”块在Python 2中不存在，我在Python 3中找不到关于它的含义的任何信息。事实证明这是正确的Python，它被解释器接受:
def f(x) -> 123:
return x
我认为这可能是一种前提语法，但是:
我不能在这里测试x，因为它仍然没有定义， 无论我在箭头后面放什么(例如2 < 1)，它都不会影响函数的行为。
熟悉这种语法风格的人能解释一下吗?
当前回答
这些是PEP 3107中包含的函数注释。具体来说，->标记返回函数注释。
例子:
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'}
注释是字典，所以你可以这样做:
>>> '{:,} {}'.format(kinetic_energy(12,30),
kinetic_energy.__annotations__['return'])
'5,400.0 Joules'
你也可以有一个python数据结构，而不仅仅是字符串:
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'
或者，你可以使用函数属性来验证调用的值:
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)
打印
>>> f(2,2)
AssertionError: x==2; Test: _between: must be between 3 and 10
>>> f(3,2.1)
AssertionError: y==2.1; Test: <lambda>
其他回答
def f(x) -> 123:
return x
我的总结:
Simply -> is introduced to get developers to optionally specify the return type of the function. See Python Enhancement Proposal 3107 This is an indication of how things may develop in future as Python is adopted extensively - an indication towards strong typing - this is my personal observation. You can specify types for arguments as well. Specifying return type of the functions and arguments will help in reducing logical errors and improving code enhancements. You can have expressions as return type (for both at function and parameter level) and the result of the expressions can be accessed via annotations object's 'return' attribute. annotations will be empty for the expression/return value for lambda inline functions.
这些是PEP 3107中包含的函数注释。具体来说，->标记返回函数注释。
例子:
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'}
注释是字典，所以你可以这样做:
>>> '{:,} {}'.format(kinetic_energy(12,30),
kinetic_energy.__annotations__['return'])
'5,400.0 Joules'
你也可以有一个python数据结构，而不仅仅是字符串:
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'
或者，你可以使用函数属性来验证调用的值:
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)
打印
>>> f(2,2)
AssertionError: x==2; Test: _between: must be between 3 and 10
>>> f(3,2.1)
AssertionError: y==2.1; Test: <lambda>
请参考PEP3107规范。这些是函数注释。Python 2。X有文档字符串。类似地，Python 3引入了->作为函数注释的使用。Python在生成文档时使用这些。
它只是告诉用户它期望什么或返回什么值
funcname。__annotations__将打印细节
like
def function(name:str ,age:int) -> "printing the personal details ":
print(f"name is {name} age is {age}")
function("test",20)
print(function.__annotations__)
输出
name is test age is 20
{'name': <class 'str'>, 'age': <class 'int'>, 'return': 'printing the personal details '}
即使您返回值，它也不显示任何内容。
这意味着函数返回的结果类型，但它可以是None。
它在面向Python 3.x的现代库中广泛存在。
例如，它在很多地方的库panda -profiling代码中有:
def get_description(self) -> dict:
def get_rejected_variables(self, threshold: float = 0.9) -> list:
def to_file(self, output_file: Path or str, silent: bool = True) -> None:
"""Write the report to a file.
