Python 全局变量和类功能
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Python global variable and class functionality
提问by user2318861
Im creating a simple python program that gives basic functionality of an SMS_Inbox. I have created an SMS_Inbox method.
我创建了一个简单的 python 程序，它提供了 SMS_Inbox 的基本功能。我创建了一个 SMS_Inbox 方法。
store = []
message_count = 0
class sms_store:
def add_new_arrival(self,number,time,text):
store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
**message_count += 1**
def delete(self,i):
if i > len(store-1):
print("Index does not exist")
else:
del store[i]
message_count -= 1
In the bolded bit I am getting an error:
在粗体部分，我收到一个错误：
UnboundLocalError: local variable 'message_count' referenced before assignment.
I created a global variable store which is an empty list and this works when I use the add_new_variable object. However for some reason it is not adding values to my global message_count variable.
我创建了一个全局变量存储，它是一个空列表，这在我使用 add_new_variable 对象时有效。但是由于某种原因，它没有向我的全局 message_count 变量添加值。
Please help
请帮忙
采纳答案by Eric
That's not how classes work. Data should be stored within the class instance, not globally.
这不是类的工作方式。数据应该存储在类实例中，而不是全局存储。
class SMSStore(object):
def __init__(self):
self.store = []
self.message_count = 0
def add_new_arrival(self,number,time,text):
self.store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
self.message_count += 1
def delete(self, i):
if i >= len(store):
raise IndexError
else:
del self.store[i]
self.message_count -= 1
sms_store = SMSStore()
sms_store.add_new_arrival("1234", "now", "lorem ipsum")
try:
sms_store.delete(20)
except IndexError:
print("Index does not exist")
print sms_store.store
# multiple separate stores
sms_store2 = SMSStore()
sms_store2.add_new_arrival("4321", "then", "lorem ipsum")
print sms_store2.store
回答by Martijn Pieters
You are trying to assign to a global variable message_countwithout declaring it as such:
您试图分配给一个全局变量message_count而不声明它：
message_count = 0
class sms_store:
def add_new_arrival(self,number,time,text):
store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
global message_count
message_count += 1
Try to avoid using globals, or at least encapsulate the variable as a classattribute:
尽量避免使用全局变量，或者至少将变量封装为类属性：
class sms_store:
message_count = 0
store = []
def add_new_arrival(self,number,time,text):
sms_store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
sms_store.message_count += 1
However, your class instanceshave no state anymore, so there is no pointin creating a class here. It only serves to confuse your purpose.
然而，你的类实例已经没有状态了，所以在这里创建一个类是没有意义的。它只会混淆你的目的。
Either store state in instances, or use global functions (so don't use a class at all); the former is preferable to the latter.
要么在实例中存储状态，要么使用全局函数（所以根本不要使用类）；前者优于后者。
Transforming your setup to a class whose instances hold state, using proper PEP-8 styleguide naming and string formatting:
使用正确的 PEP-8 样式指南命名和字符串格式，将您的设置转换为实例保持状态的类：
class SMSStore(object):
def __init__(self):
self.store = []
self.message_count = 0
def add_new_arrival(self, number, time, text):
self.store.append('From: {}, Received: {}, Msg: {}'.format(number, time, text))
self.message_count += 1
You are then free to create oneinstance and use that as a global if needs be:
然后，您可以自由创建一个实例并将其用作全局实例，如果需要：
sms_store = SMSStore()
Other code just uses sms_store.add_new_arrival(...), but the state is encapsulated in one instance.
其他代码只是使用sms_store.add_new_arrival(...)，但状态被封装在一个实例中。
回答by Sukrit Kalra
If the variable you are referring to is message_count, the error is because in Python, you have to specify a variable as globalbefore you can make edits with it.
如果您所指的变量是message_count，则错误是因为在 Python 中，您必须像global之前一样指定一个变量，然后才能对其进行编辑。
This should work.
这应该有效。
store = []
message_count = 0
class sms_store:
def add_new_arrival(self,number,time,text):
global message_count
store.append(("From: "+number, "Recieved: "+time,"Msg: "+text))
message_count += 1
def delete(self,i):
if i > len(store-1):
print("Index does not exist")
else:
global message_count
del store[i]
message_count -= 1
As written above, you'd be better off encapsulating it in the __init__function instead of declaring it global.
如上所述，最好将它封装在__init__函数中而不是声明它global。
