Set the name of a python object/variable with a string

问题

Most people will probably say this is a bad idea. I want to use the content of a string as the name of a variable. I want to accomplish the dreaded:

s = 'x'
x = 1

where the variable name x comes (somehow?) from the string s.

To answer the "why?", say I have a global default value for x that I want the option of overriding in a function. But:

x = 0
def f(**kw):
    print x
f(x=1)

prints 0 not 1. If I could use the strings in kw.keys() to reassign x (or any other globally set variables) then I'd be happy.

I realize that this works for reassigning x in f:

x = 0
def f(x=x):
    print x
f(x=1)

But I want to do this for cases where there are MANY variables in my namespace that I might at some point want to override without rewriting every function definition in my module.

回答1:

Check out exec

>>> print x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
>>> s = 'x'
>>> exec(s + " = 1")
>>> print x
1 

See also: How can I assign the value of a variable using eval in python?

After a little experimentation, this also seems to work:

>>> print x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
>>> s = 'x'
>>> globals()[s] = 1
>>> print x
1

回答2:

You should reconsider using the global keyword (http://docs.python.org/2.7/reference/simple_stmts.html#the-global-statement)

I recommend taking a look at Use of "global" keyword in Python as well.

Also, as promanow suggested, using a global mutable object might be a better approach.

However, keep in mind that having large amounts of code that might modify or depend on a mutating global is very scary idea, from a maintenance perspective. Tread carefully.

回答3:

Assignment via function arguments is not obvious and therefore is not considered Pythonic (Python is not C). See import this.

The cheap way that I've done flat configuration is through a global partial dict:

In [1]: from functools import partial
In [2]: config = partial(dict)
In [3]: def f(x=None):
   ...:     print x or config.x
   ...:     
In [4]: config.x = 'foo'
In [5]: f()
foo

Something to this effect is obvious, readable and therefore much more maintainable.

回答4:

But I want to do this for cases where there are MANY variables in my namespace that I might at some point want to override without rewriting every function definition in my module.

It sounds like you want a class:

class Foo(object):
    x = 0
    def f(self):
        print self.x

my_foo = Foo()
my_foo.x = 1
my_foo.f()

来源：https://stackoverflow.com/questions/14764088/set-the-name-of-a-python-object-variable-with-a-string