I'm having terrible trouble trying to understand python scoping rules.
With the following script:
a = 7 def printA(): print "Value of a is %d" % (a) def setA(value): a = value print "Inside setA, a is now %d" %(a) print "Before setA" printA() setA(42) print "After setA" printA()
Gives the unexpected (to me) output of:
Before setA Value of a is 7 Inside setA, a is now 42 After setA Value of a is 7
Where I would expect the last printing of the value of a to be 42, not 7. What am I missing about Python's scope rules for the scoping of global variables?
Use of “global†keyword to modify global variable inside a function. If your function has a local variable with same name as global variable and you want to modify the global variable inside function then use 'global' keyword before the variable name at start of function i.e.
Global variables are those which are not defined inside any function and have a global scope whereas local variables are those which are defined inside a function and its scope is limited to that function only.
Python has no command for declaring a variable. A variable is created when some value is assigned to it. The value assigned to a variable determines the data type of that variable. Thus, declaring a variable in Python is very simple.
The reason the first alert is undefined is because you re-declared global as a local variable below it in the function. And in javascript that means from the top of the function it is considered the local variable.
Global variables are special. If you try to assign to a variable a = value
inside of a function, it creates a new local variable inside the function, even if there is a global variable with the same name. To instead access the global variable, add a global
statement inside the function:
a = 7 def setA(value): global a # declare a to be a global a = value # this sets the global value of a
See also Naming and binding for a detailed explanation of Python's naming and binding rules.
The trick to understanding this is that when you assign to a variable, using =, you also declare it as a local variable. So instead of changing the value of the global variable a, setA(value) actually sets a local variable (which happens to be called a) to the value passed in.
This becomes more obvious if you try to print the value of a at the start of setA(value) like so:
def setA(value): print "Before assignment, a is %d" % (a) a = value print "Inside setA, a is now %d" % (a)
If you try to run this Python will give you a helpful error:
Traceback (most recent call last): File "scopeTest.py", line 14, in setA(42) File "scopeTest.py", line 7, in setA print "Before assignment, a is %d" % (a) UnboundLocalError: local variable 'a' referenced before assignment
This tells us that Python has decided that the setA(value) function has a local variable called a, which is what you alter when you assign to it in the function. If you don't assign to a in the function (as with printA()) then Python uses the global variable A.
To mark a variable as global you need to use the global keyword in Python, in the scope that you want to use the global variable. In this case that is within the setA(value) function. So the script becomes:
a = 7 def printA(): print "Value of a is %d" % (a) def setA(value): global a a = value print "Inside setA, a is now %d" %(a) print "Before setA" printA() setA(42) print "After setA" printA()
This one line addition tells Python that when you use the variable a in the setA(value) function that you are talking about the global variable, not a local variable.
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