[SOLVED] Recommended way to declare global variable?

[Winfried]

Hello,

To avoid the UnboundLocalError, out of curiosity, can't a global variable be declared somehow within an objet instead of at the top level and having to use "global"?

Thank you.

URL = "https://www.acme.com"

class MyFrame(wx.Frame):
	#No change
	#URL = "https://www.acme.com"
	
	def Dload(self):
		#UnboundLocalError: cannot access local variable 'URL' where it is not associated with a value
		#response = requests.get(URL)

		#No change
		#response = requests.get(self.URL)

		#fixed. Is there a better way?
		global URL
		response = requests.get(URL)
		...

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Edit: Is there a way to avoid Cloudfare's annoying "Performing security verification - This website uses a security service to protect against malicious bots. This page is displayed while the website verifies you are not a bot." ?

--
Edit: Turns out this does work:

import wx

class ListBoxFrame(wx.Frame):
	URL = "https://www.acme.com"

	def Dload(self):
		print("URL/2:",self.URL)

	def __init__(self, *args, **kwargs):  
		super().__init__(None, wx.ID_ANY,size=(1000,400), title='blah')

		print("URL/1:",self.URL)
		self.Dload()
		
		panel = wx.Panel(self, -1)
		panel.Layout()

app = wx.App()
ListBoxFrame().Show()
app.MainLoop()

[deanhystad]

There's no need for the "global" in this example.

URL = "https://www.acme.com"
 
class MyFrame(wx.Frame):
    def Dload(self):
        global URL
        response = requests.get(URL)

Python will first look for URL as a local variable (assigned in Dload), then it will look for URL in the global name space (defined in the module, outside a class definition). global would be needed if Dload assigned a value to URL.


In this example URL is not a global variable, it is a class variable.

class ListBoxFrame(wx.Frame):
    URL = "https://www.acme.com"

To access URL, you need to use a class reference

print(ListBoxFrame.URL)

You can also use an instance (like self) to reference URL. Python first looks for an instance variable named URL, then looks for a class variable.

As with the global variable example, a class variable acts differently when doing assignment. This code might produce unexpected results.

class Example:
    URL = "https://www.acme.com"
 
    def set_url(self, url):
        self.URL = url

a = Example()
b = Example()
b.set_url("not a url")
print(a.URL, Example.URL, b.URL)

Output:
https://www.acme.com https://www.acme.com not a url

The set_url method does not modify the class variable URL. It creates a new instance variable named URL. The class URL and instance URL are different objects and can have different values. The class URL is visible to all ListBoxFrame instances. The instance URL variable only visible to ListBoxFrame instances where set_url() was called. Such behavior is normally considered a bug.

You asked for recommendations on how to declare a global variable. My advice is to not use global variables. I would write you example like this.

import wx
 
class ListBoxFrame(wx.Frame):
    def Dload(self):
        print("URL/2:",self.url)
 
    def __init__(self, url, *args, **kwargs):  
        super().__init__(None, wx.ID_ANY,size=(1000,400), title='blah')
        self.url = url
        self.Dload()
        panel = wx.Panel(self, -1)
        panel.Layout()
 
app = wx.App()
ListBoxFrame("https://www.acme.com").Show()
app.MainLoop()

If ListBoxFrame can only work using www.acme.com I would consider making URL a class variable (and changing the class name to indicate its limited reusability).