It'a simple example.

```
d=pd.DataFrame({'a':[1,2,3,4],
                'b':[6,6,7,8]})

```

I write a function f(x).I want to print d.x. The function is:

```
def f(x):
  print(d.x)

```

Run `f(a)`. I expect it to print `d.a`

But it returns `NameError: name 'a' is not defined`.
How do I solve it?