Python check if named is defined

How to check if variable exists in Python

Objective: This article will discuss the different methods to check if a variable exists in Python.

Before we dive into the methods to check for the availability of a variable in the code, let us first understand why we need to do so? 🤔

To answer the above question, you must understand “what is a NameError and an UnboundLocalError in Python?“

🐞 NameError and UnboundLocalError in Python

📌 In Python, if you try to access or interact with a variable name that is not defined, you get NameError: name ‘variable_name’ is not defined exception.

Traceback (most recent call last):
File “main.py”, line 5, in
add_up += 1
NameError: name ‘add_up’ is not defined

❖ Since the variable with name ‘add_up’ is not defined in our code in the above example, we got a NameError .

📌 Similarly, you get UnboundLocalError: local variable ‘variable_name’ referenced before assignment exception when you refer a local variable before assigning it in your program.

Traceback (most recent call last):
File “main.py”, line 10, in
print(‘Average = ‘, total(numbers))
File “main.py”, line 5, in total
avg += x / l
UnboundLocalError: local variable ‘avg’ referenced before assignment

❖ We got UnboundLocalError exception because the local variable with the name ‘avg’ was referenced before being assigned in the program.

⚡ Thus, the aforementioned exceptions occur primarily due to the following reasons:-
➥ If you forget to define a variable.
➥ If you use the wrong variable name.
➥ Or if you try to access a variable before defining it.

Both of these errors are pretty common, especially for newbies. To fix them, you can simply edit your program and manually define the variable that leads to the occurrence of these errors.

However, if your code is quite lengthy, then it can be extremely tedious to spot every undefined variable one by one in your program. This is where you can check if a variable exists in the first place before using it.

Hence, without further delay, let us discuss the methods to check if a variable exists in the Python code.

✨ Method 1: Using locals() and globals()

locals() and globals() are built-in functions in Python that can be used to check for the availability of an identifier. Both these functions return the local and global symbol table, respectively. Thus, you can simply check if a variable exists locally or globally using the respective symbol tables.

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Check if a variable is defined in Python

This post will discuss how to check if a variable is defined in Python.

1. Using locals() function

To check if the variable is defined in a local scope, you can use the locals() function, which returns a dictionary representing the current local symbol table.

2. Using vars() function

Alternatively, you can use no-arg vars() function, which acts like locals() function.

3. Using dir() function

Without arguments, the dir() function returns the list of names in the current local scope.

4. Using globals() function

Finally, to check for the existence of a global variable, you can use the globals() function. It returns a dictionary representing the current global symbol table.

That’s all about determining whether a variable is defined in Python.

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Check if name is defined?

Dennis Sakva 1429

If you don’t want to use try / except , you could lookup the locals() and globals() . Such a check would look like:

if locals().get('c', globals().get('c')) is None: print "no" else: print "ok" 

The call in the if condition would lookup the local variables first, and if your variable isn’t found there, would then lookup global variables. If the variable isn’t found in one or the other, None is returned

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Check whether a variable is defined or not in Python

The topic deals with the Python variables. Here we are going to determine if a Python variable exists or not.

Certainly, a variable is a container of data. In other words, we can store our data in a variable that we can use for our task.

Defining a Variable :

Data or information needs to be stored for future operations. Moreover, Variable gives the facility to store data.

Let’s see how we define a variable in Python.

a = 4 # a is the variable which stores some data b = 12.9 c = "Hello" # Defining a variable print (a,b,c) # printing the values of the variable

Now let’s continue to our topic without wasting any more time…

Check if a variable is defined or not in Python :

A very simple method to check,

Certainly, this is the topic to be dealt with. So let’s see below:

• First of all, if a variable is defined, it would have some data in it.
• If it is not defined then no data is there in it.

The following example will clear the concept regarding this,

a = 4 # a is the variable which stores some data b = 12.9 c = "Hello" # Defining a variable print (c) # c is defined so no error is raised print (d) # d is not defined so error is raised

Output : Hello Traceback (most recent call last): File "main.py", line 10, in print (d) NameError: name 'd' is not defined

So we can see, that there will be an error appear if our variable is not defined.

We can check the existence of a variable using the Python try and except can be used to deal with the following conditions.

• The code is written in the try block.
• If any kind of error arises, then statements in except block get executed.
• If there is no error, except block statements, they are not executed.

The following example may clarify the doubts regarding the use of try/except if any,

a = 10 # a is defined try: print (c) # variable c is not defined. # therefore error raises and control shifts to except block. except: print ("Variable is not defined")

Output : Variable is not defined

This is the way to determine or check whether a variable is defined or not in Python.

2 responses to “Check whether a variable is defined or not in Python”

a “try: .. except: ” block is pretty expensiv for just checking if a variable is defined, specially if it is checked like this in a loop. normally you can check it with:
“`python
if ‘c’ in dir():
print (c)
else:
print (“Variable is not defined”)
“`
to check if a variable is in a class dir() or dir().

It’s title is not about how to define variable. Just how to check if already defined. Few strings enough instead of article. As a result: TL;DR;

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