To update a global variable in one line of Python, retrieve the global variable dictionary with the
globals() function, and access the variable by passing the variable name as a string key such as
globals()['variable']. Then overwrite the global variable using the equal symbol, for example in
globals()['variable'] = 42 to overwrite the variable with value
Let’s dive into this mission-critical challenge in greater detail!
Problem: The common approach to update a global variable takes two lines within the inner scope (in our example the function
a = 42 def update(): global a a = 21 update() print(a) # 21
Is this really necessary? Can’t you update the global variable
a in only a single line of code?
You may want to look for something like this:
# WRONG: global a = 21
But this is incorrect syntax in Python. How to update the global variable in a single line?
Let’s dive into multiple methods to accomplish this. First, try them yourself in our interactive Python shell:
Exercise: Do all four ways update the global variable?
Method 1: Semicolon
def update_1(): global a; a = 21
The semicolon allows you to write multiple Python expressions in a single line and executing them in sequence.
Related article: How to execute multiple lines of Python in a single line?
Method 2: sys.modules[__name__]
# Method 2 import sys this = sys.modules[__name__] this.a = 42 def update_2(): this.a = 21
The code goes through the following steps:
- Import the
- Assign the module in which you run this code to the variable
- Access the global variable
athrough the dot notation
By doing this, you work with qualified variable names (bare names) rather than implicit variable names. This reduces the likelihood of mistakes and is a clean way to solve this problem. This way, you can avoid using the
global keyword altogether which can improve readability of your code.
Method 3: globals()[var]
def update_3(): globals()['a'] = 21
The code first accesses all global variables using the
globals() function that returns a dictionary mapping names to objects. You access the value associated to the key
'a'. The return value is the object to which global variable
Method 4: Semicolon
def update_4(): globals().update(a=21)
This code is very similar to the previous code in Method 3. You use the dictionary
update() instead of the indexing method though to overwrite the value of global variable
Where to Go From Here?
Enough theory, let’s get some practice!
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