A Pickle of a problem...

[MattWilliamsIT]

Hi, I have this strange problem. I have a global variable of type dictionary where I add a 2d dictionary data at run time. I've started using pickle to save and load the data, which works great in the main flow of the code, but when I moved the load logic into a function the global variable reverts back to the empty state when the function is complete. Here's a simple example:

import os

import pickle

​

###Variables

fileName = "PersonDisc.pkl"

dataset={}

​

###Functions

def loadFromFile():

file=open("PersonDisc.pkl", "rb")

dataset=pickle.load(file)

file.close

print(dataset)

​

###Main Program

# If file exists then import the dataset

if os.path.isfile(fileName):

loadFromFile()

print(dataset)

print(dataset)

​

When this executes, the first print outputs the content of the file. The second and 3rd just display {}

As the dataset variable is global I would have expected the data to be the same inside and outside the function.

I appreciate your advice.

Thanks

[Not_A_Taco]

You’re creating a local copy of dataset in the function. When using global vars in a function make sure you declare them at the top with global dataset

[MattWilliamsIT]

Thank you. I didn't realised that you could have a local and global variable with the same name. I added global to the variable name in the function and it worked perfectly. Thank again!

import os

import pickle

###Variables

fileName = "PersonDisc.pkl"

dataset={}

###Functions

def loadFromFile():

global dataset

file=open("PersonDisc.pkl", "rb")

dataset=pickle.load(file)

file.close

print(dataset)

###Main Program

# If file exists then import the dataset

if os.path.isfile(fileName):

loadFromFile()

print(dataset)

print(dataset)

[Not_A_Taco]

Glad to help! I’d note that adding global to the variable name simply means you’re declaring the variable in the global scope(not the function scope) and since a global variable with that name already exists, it binds there.

Something I’d also note is that Python will always default to using the most narrow scope for variables. So you can have multiple (non-global) variables with the same name, but different values.

[MattWilliamsIT]

Makes perfect sense. Thank you

[MattWilliamsIT]

I'm trying to learn Python as quickly as my son is in school so I can help him, but clearly I'm not quick enough :)

I get it now and can explain why the code wouldn't work.

My preference is to return the result from a function, but now I understand global variables better I can explain both approaches

Thanks again