Criar menu de PyQt a partir de uma lista de strings

Question

Eu tenho uma lista de strings e deseja criar uma entrada de menu para cada uma dessas cordas. Quando o usuário clica em uma das entradas, sempre a mesma função será chamada com a string como um argumento. Depois de alguma tentativa e pesquisa eu vim com algo como isto:

import sys
from PyQt4 import QtGui, QtCore

class MainWindow(QtGui.QMainWindow):
    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.menubar = self.menuBar()
        menuitems = ["Item 1","Item 2","Item 3"]
        menu = self.menubar.addMenu('&Stuff')
        for item in menuitems:
            entry = menu.addAction(item)
            self.connect(entry,QtCore.SIGNAL('triggered()'), lambda: self.doStuff(item))
            menu.addAction(entry)
        print "init done"

    def doStuff(self, item):
        print item

app = QtGui.QApplication(sys.argv)
main = MainWindow()
main.show()
sys.exit(app.exec_())

Agora o problema é que cada um dos itens do menu irá imprimir a mesma saída: "Item 3" em vez do correspondente. Eu sou grato por quaisquer ideias sobre como posso obter este direito. Obrigado.

Answer 1

You're meeting what's been often referred to (maybe not entirely pedantically-correctly;-) as the "scoping problem" in Python -- the binding is late (lexical lookup at call-time) while you'd like it early (at def-time). So where you now have:

    for item in menuitems:
        entry = menu.addAction(item)
        self.connect(entry,QtCore.SIGNAL('triggered()'), lambda: self.doStuff(item))

try instead:

    for item in menuitems:
        entry = menu.addAction(item)
        self.connect(entry,QtCore.SIGNAL('triggered()'), lambda item=item: self.doStuff(item))

This "anticipates" the binding, since default values (as the item one here) get computed once an for all at def-time. Adding one level of function nesting (e.g. a double lambda) works too, but it's a bit of an overkill here!-)

You could alternatively use functools.partial(self.doStuff, item) (with an import functools at the top of course) which is another fine solution, but I think I'd go for the simplest (and most common) "fake default-value for argument" idiom.

Answer 2

This should work, but I'm pretty sure there was a better way that I can't recall right now.

def do_stuff_caller(self, item):
    return lambda: self.doStuff(item)

...
self.connect(entry, QtCore.SIGNAL('triggered()'), self.do_stuff_caller(item))

Edit: Shorter version, that still isn't what I'm thinking about... or maybe it was in another language? :)

(lambda x: lambda self.do_stuff(x))(item)