问题
I have a Python object with attributes a, b, c.
I still use old string formatting, so I'd normally print these manually:
print 'My object has strings a=%s, b=%s, c=%s' % (obj.a, obj.b, obj.c)
Lately, my strings have been getting super long, and I'd much rather be able to simply pass the object into a string format function, something like:
print 'My object has strings a=%a, b=%b, c=%c'.format(obj)
However, the syntax is incorrect. Is this possible?
回答1:
You can use the .attribute_name notation inside the format fields themselves:
print 'My object has strings a={0.a}, b={0.b}, c={0.c}'.format(obj)
Below is a demonstration:
>>> class Test(object):
... def __init__(self, a, b, c):
... self.a = a
... self.b = b
... self.c = c
...
>>> obj = Test(1, 2, 3)
>>> 'My object has strings a={0.a}, b={0.b}, c={0.c}'.format(obj)
'My object has strings a=1, b=2, c=3'
>>>
Note however that you do need to number the format fields when doing this. Also, as you can see, the str.format function has its format fields denoted by curly braces {...}, not the % sign.
For more information, here is a reference on the Format String Syntax in Python.
回答2:
I think it's preferable to use vars() to access an object's attributes as a dict rather than usng __dict__.
So you could do this:
"My object has strings a={a}, b={b}, c={c}".format(**vars(obj))
For more background on why vars() is preferable to __dict__, see the answer to the question Use dict or vars()?.
回答3:
As @igniteflow wrote in a buried comment:
'My object has strings a={a}, b={b}, c={c}'.format(**obj.__dict__)
With my limited python understanding: .__dict__ is a dict with all the instances attributes and the ** operator basically unpacks them and adds them as key=value args to the method
回答4:
Some of the suggested answers that rely on vars(...) don't work with immutable objects:
>>> class foo(object):
... __slots__ = ()
... def __init__(self):
... self.x = 1
...
>>> vars(foo())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in __init__
AttributeError: 'foo' object has no attribute 'x'
I think it's preferable to stick with something reliable and explicitly list what you need to be printed.
If you need to print a ton of attributes then storing those values as attributes might not be the best idea.
回答5:
For the sake of completeness, building on @igniteflow and @Christian, you could use the % string format operator and write:
'My object has strings a=%(a)s, b=%(b)s, c=%(c)s' % obj.__dict__
来源：https://stackoverflow.com/questions/26791908/use-str-format-to-access-object-attributes