CODEWITHSUNDEEP
python
Wizard
Wizard

Reputation: 22113

Why assign to None prompts that variable is not defined

I created a snippet of code to test the variable declaration,

In [37]: price = {"apple": 3, "orange":1}                                                                         

In [38]: for key in price: 
    ...:     fruit = key                                                                                          
In [39]: fruit                                                                                                    
Out[39]: 'orange'

It works without efforts to declare variable fruit in advance,
Nonetheless, 

In [44]: cars = {}                                                                                                

In [45]: for key in cars: 
    ...:     car = key                                                                                            
In [46]: car                                                                                                      
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-46-38f6ffefbd69> in <module>
----> 1 car

NameError: name 'car' is not defined

It prompt car is not defined, 

In [51]: for key in cars: 
    ...:     print(type(car), dir(car))  
#it output nothing, key is None

but None can be assigned to car 

In [52]: car = None                                                                                               
In [53]: car                                                                                                     
In [54]: i = car                                                                                                  
In [55]: i

It is not report error that i is not defined,

What's the difference of the two cases?

Upvotes: 0

Views: 79

Answers (4)

Austin
Austin

Reputation: 26037

price = {"apple": 3, "orange":1}

for key in price:
    fruit = key

print(fruit)

This one iterates through keys in dictionary. When you simply do a loop through dictionary (like for x in dictionary), you iterate through its keys. In your loop, you are assigning key to the same variable not a list or any other data structure, so you replace everytime in the loop.

cars = {}     

for key in cars: 
    car = key

print(car)

Here, for loop does not iterate since dictionary is empty so Python can't identify what car is.

Dictionary empty means there is no item inside it, not even None:

cars = {}                              
print(cars)
# {}

Upvotes: 1

LeKhan9
LeKhan9

Reputation: 1350

Cars does not have any keys in it (no pun intended), so the statement car = key will never be executed.

For the case with i = car, well i just becomes a reference to the same location that car is pointing to when you state i = car, so its not a surprise it will return the same value as car.

Upvotes: 1

Sraw
Sraw

Reputation: 20224

You misunderstand how it works.

You have an empty dict, there is even no None in it. And iterating an empty collection is just ignored. So the for loop doesn't execute at all.

Python is a complete OO language which means every element in it is an object. So is None. You can add None into a collection. It will show you there is a None. But in your case, there is just nothing.

Upvotes: 1

Genome
Genome

Reputation: 1236

The difference is variable initialization. For example...

car = None

...sets the "car" variable to None. In the first example outlined, on line 46 "car" is not defined because the loop never iterates and variable is never initialized (since "cars" is an empty dict).

Upvotes: 1

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