creating a custom widget in tkinter

Your widget should subclass Frame. Within the frame you can use any geometry manager you want without affecting any other code. It's important that the widget class does not call grid, pack or place on itself -- that's the job of the function that creates the widget. Every widget, or function that creates a widget, should only ever worry about laying out its children.

Here's an example that creates a couple of different custom widgets. Each uses a different geometry manager to illustrate that they don't interfere with each other:

try:
    # python 3.x
    import tkinter as tk
except ImportError:
    # python 2.x
    import Tkinter as tk


class CustomWidget(tk.Frame):
    def __init__(self, parent, label, default=""):
        tk.Frame.__init__(self, parent)

        self.label = tk.Label(self, text=label, anchor="w")
        self.entry = tk.Entry(self)
        self.entry.insert(0, default)

        self.label.pack(side="top", fill="x")
        self.entry.pack(side="bottom", fill="x", padx=4)

    def get(self):
        return self.entry.get()

class Example(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
        self.label = tk.Label(self)
        self.e1 = CustomWidget(self, "First Name:", "Inigo")
        self.e2 = CustomWidget(self, "Last Name:", "Montoya")
        self.submitButton = tk.Button(self, text="Submit", command=self.submit)

        self.e1.grid(row=0, column=0, sticky="ew")
        self.e2.grid(row=1, column=0, sticky="ew")
        self.label.grid(row=2, column=0, sticky="ew")
        self.submitButton.grid(row=4, column=0)

        self.grid_columnconfigure(0, weight=1)
        self.grid_rowconfigure(2, weight=1)

    def submit(self):
        first = self.e1.get()
        last = self.e2.get()
        self.label.configure(text="Hello, %s %s" % (first, last))

if __name__ == "__main__":
    root = tk.Tk()
    Example(root).place(x=0, y=0, relwidth=1, relheight=1)
    root.mainloop()