如何在 Python 中动态生成字符串（观察者模式）

Question

Supposing we have the code below:

var1="top"
var2=var1+"bottom"

We want to change var1 value if a condition is true:

if COND==True:
  var1="changed"

Now I want to have var2 dynamically changed . With the code above, var2 will still have the value " topbottom ".

How can I do that?

Thanks

Answer 1

You can elegantly achieve this with a callback proxy from ProxyTypes package:

>>> from peak.util.proxies import CallbackProxy
>>> var2 = CallbackProxy(lambda: var1+"bottom")
>>> var1 = "top"
>>> var2
'topbottom'
>>> var1 = "left"
>>> var2
'leftbottom'

Each time you access your var2 , callback lambda will be executed and a dynamically generated value returned.

Answer 2

You can use string formatting to specify a placeholder in var2 where you want the updated value of var1 to be placed:

In [1653]: var2 = '{}bottom'

The {} brackets here specify a placeholder. Then call var2.format to insert var1 into var2 as and when needed.

In [1654]: var1 = 'top'

In [1655]: var2.format(var1)
Out[1655]: 'topbottom'

In [1656]: var1 = 'changed'

In [1657]: var2.format(var1)
Out[1657]: 'changedbottom'

Answer 3

There is no simple way to do this as string are immutable in python. There is no way var2 can be changed after var1+"bottom" evaluation. You either need to create a new string (not sure why do don't want to do this) or you need to write your own class and create your own objects that accept this behavior. If you want to do this, take a look at Observer Pattern

Answer 4

As others have said, since strings are immutable you must find a way to insert the dynamic value on the formation of the string.

I am a fan of the new 'fstrings' in Python- single line 'if' statement for flare:

cond = True

var1 = "changed" if cond is True else "top"
var2 = f"{var1} bottom"

print(var2)