Error message when opening new window in Tkinter

You should define the function before you reference it.

>>> def f():...     convert_c2d(1)...     def convert_c2d(n):...         pass...>>> f()Traceback (most recent call last):  File "<stdin>", line 1, in <module>  File "<stdin>", line 2, in fUnboundLocalError: local variable 'convert_c2d' referenced before assignment>>> def g():...     def convert_c2d(n):...         pass...     convert_c2d(1)...>>> g()>>>

Simply change of the codes as below will solve your problem:

def convert_c2d(n):    if not n.get():        return res.set("Insert a numeric value.")    try:        c = float(n.get())        diameter = c/pi        return res.set(str(diameter))    except ValueError:        return res.set("Insert a numeric value.")convert = tk.Button(c2d, text="Convert", command=convert_c2d, activeforeground="red")convert.grid(row=2, column=0, columnspan=2, pady=2, padx=2, sticky=E+W+N+S)

UPDATE

convert_c2d takes a parameter. But tkinter does not call the command callback with argument. You should pass it using another function (or lambda as below):

convert = tk.Button(c2d, text="Convert", command=lambda: convert_c2d(entry_c),                    activeforeground="red")

or you can change the convert_c2d not to take any argument, and make the function directly access entry_c. (nested function can access the variable from enclosing scope).