请问一下,python中,组件entty默认值不是为空吗?
```python
import tkinter
from tkinter.messagebox import *
class hx():
d_z=None
def ddz(self,dz):
if dz==None:
showwarning("警告","地址不能为空!")
else:
self.d_z=dz
print(self.d_z)
a=hx()
win=tkinter.Tk()
win.title("孔网统计2.0")
l1=tkinter.Label(win,text="数据库地址:").grid(row=0,column=0)
d=tkinter.StringVar()
e1=tkinter.Entry(win,textvariable=d).grid(row=0,column=1)
b1=tkinter.Button(win,text="确定",command=lambda:a.ddz(d)).grid(row=0,column=3)
win.mainloop()
```