Why assign to None prompts that variable is not defined

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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?

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)
# {}

Python Variables and Assignment, In Python, variables do not require forward declaration - all you need to do is and hit "Enter", the interpreter will just show you a new line prompt. Space is still allocated for the variable - only the value is set to None . UnboundLocalError: local variable 'lives' referenced before assignment. python variables python-3.5. Share a link to this question. improve this question. edited Jun 21 '16 at 21:55. asked Jun 21 '16 at 21:43. 7 bronze badges. Take a look at @chepner's answer. The global declaration should be in the main function. If you do that, your code

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).

Syntactic Effects of Conjunctivist Semantics: Unifying Movement , to mean that the thing x is one of the things X, then the formula is true, because The two interpretations of second- order variables therefore can not be mere This prompts a corresponding adjustment in what constitutes an assignment of are no “leftovers” among theseX that bear the Internal relation to none of thoseY,​  You'll receive the prompt you got. You won't run into problems if you keep it that way but it's bad practice. In those two cases you are using a function to change the variable: list() and append() .

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.

2. Variables, expressions and statements, The assignment statement creates new variables and gives them values: The assignment operator, =, should not be confused with an equals sign (even The following are all legal Python expressions whose meaning is more or less clear: Each of these functions allows a prompt to be given to the function between the​  jmehnle commented on Apr 8, 2016. @radeksimko, in my understanding this ( #1899) is about terraform plan never prompting for required variables, whereas #1230 is about terraform (not just terraform plan) not prompting for required variables if a tfvars file is read. Example code passing for #1230 doesn't prove #1899 is fixed.

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.

Testing if a Variable Is Defined - Python Cookbook [Book], In Python, all variables are expected to be defined before use. The None object is a value you often assign to signify that you have no real value for a variable,  Because None is an object, we cannot use it to check if a variable exists. It is a value/object, not an operator used to check a condition. Checking if a Variable is None. There are two ways to check if a variable is None. One way can be performed by using the is keyword. Another is using the == syntax. Both comparison methods are different

Error: Unassigned variable · Issue #21659 · hashicorp/terraform , TF_ENV-var - unassigned (still not working, no root cause found as of yet); -var '​variable=value' - assigned, but not defined. For the second case,  If you've registered/set_fact a variable YOU'VE defined it, even if the YAML/JSON value is null or if the variable is a field of a bigger object. 2.-Do NOT compare against the "None" string, just evaluate the variable.

Artificial Lighting for Photography, Definition: Several manufacturers make a variation of their standard light stand This variable leg allows placing the light stand in positions where the floor or terrain is are available with a R.M. leg, although this option is not listed in their catalog. Definition: Hi-boys, sky-hi boys, tall boys, and overhead roller stand None  The value 1 was assigned to the variable one. This is not apparent from the blank line output. But the interpreter can show you the value of any variable if you just type the variable name and hit enter: 1 2 3 >>> one 1 >>>. python. The value 1 is shown because Python evaluates the line and reports the value returned.

Python Tutorial, Python associates types with the objects, not the variables, i.e., a variable can hold python3 grade_statistics.py (All Platforms) """ # Define all the functions before using This example prompts user for a hexadecimal (hex) string, and print its Default is whitespaces '\t\n '. str.split(sep=None, maxsplit=-1) # Return a list of  with Dim ws As Worksheet you only declared a variable named ws being of type Worksheet. but you didn't initialize it, so VBA doesn't know what actual object ws should reference. if you add set ws =ThisWorkbook.Worksheets ("Sheet2") statement, then the ws variable is initialized to reference a real object (i.e. the Worksheet named "Sheet2" in

Comments
  • HINT: You are looping over a completely empty dictionary in the latter case ...
  • so for here is a if?
  • @riderdragon, Could you make it clear what you are asking? Sorry, I didn't get it.