[Python编程(第4版)].(Programming.Python.4th.Edition).Mark.Lutz.文字版

written in Python, it is both easily customized and widely portable across window

platforms. And because it is implemented with classes, it is both a standalone program

and a reusable object library.

A Simple Calculator GUI

Before I show you how to write a full-blown calculator, though, the module shown in

Example 19-17 starts this discussion in simpler terms. It implements a limited calcu-

lator GUI, whose buttons just add text to the input field at the top in order to compose

a Python expression string. Fetching and running the string all at once produces results.

Figure 19-2 shows the window this module makes when run as a top-level script.

Example 19-17. PP4E\Lang\Calculator\calc0.py

"a simplistic calculator GUI: expressions run all at once with eval/exec"

from tkinter import *

from PP4E.Gui.Tools.widgets import frame, button, entry

class CalcGui(Frame):

def __init__(self, parent=None): # an extended frame

Frame.__init__(self, parent) # on default top-level

self.pack(expand=YES, fill=BOTH) # all parts expandable

self.master.title('Python Calculator 0.1') # 6 frames plus entry

self.master.iconname("pcalc1")

self.names = {} # namespace for variables

text = StringVar()

entry(self, TOP, text)

rows = ["abcd", "0123", "4567", "89()"]

for row in rows:

frm = frame(self, TOP)

for char in row:

button(frm, LEFT, char,

lambda char=char: text.set(text.get() + char))

frm = frame(self, TOP)

for char in "+-*/=":

button(frm, LEFT, char,

lambda char=char: text.set(text.get()+ ' ' + char + ' '))

frm = frame(self, BOTTOM)

button(frm, LEFT, 'eval', lambda: self.eval(text) )

button(frm, LEFT, 'clear', lambda: text.set('') )

def eval(self, text):

try:

text.set(str(eval(text.get(), self.names, self.names))) # was 'x'

except SyntaxError:

try:

exec(text.get(), self.names, self.names)

except:

1458 | Chapter 19: Text and Language

Do

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