Making a database with custom commands in python

Question

I'm trying to make a simple, local database using Python where I can set values, get values, etc and I keep getting this error:

#Simple Database
#Functions include      Set[name][value]
#                       Get[name]
#                       Unset[name]
#                       Numequalto[value]
#                       End
#                       Begin, Rollback, Commit

varlist = []
ops = []
class item:
        def __init__(self,name,value):
                self.name = name
                self.value = value
class db:
    def __init__(self):
            self.varlist = []
            self.ops = []
    def Set(self,nm,val):
            changed = False                 #Bool for keeping track of update
            for item in varlist:    #run through current list
                    if item.name == nm:     #If the name is found
                            item.value = val #update the value
                            changed = True  #found it
                            break                   #exit if found
            if not changed:
                    newitem = item() #Create a new one and append it
                    newitem.name = nm
                    newitem.value = val
                    varlist.append(newitem)
    def Get(key):
            for item in varlist:
                    if item.name == key:
                            return item.value
                            break

    def Unset(key):
            for item in varlist:
                    if item.name == key:
                            item.value = -1
                            break

    def Numequalto(key):
            count = 0
            for item in varlist:
                    if item.value == key:
                            count+=1
            return count

def main():
    newdb = db()
    varlist=[]
    comm = "" #prime it
    while comm.lower() != "end":
        comm = input("Command: ")
        if comm.lower() == "begin":
            print("----SESSION START---")
            while comm.lower() != "end":
                    comm = input("Command: ")
                    part = []
                    for word in comm.split():
                            part.append(word.lower())
                    print(part)
                    if part[0].lower()=="set":
                            newdb.Set(part[1],part[2])
                            print(varlist)
                    elif part[0].lower()=="get":
                            gotten = Get(part[1])
                            print(gotten)
                    elif part[0].lower()=="unset":
                            Unset(part[1])
                    elif part[0].lower()=="numequalto":
                            numequal = Numequalto(part[1])
                            print(numequal)
            print("Finished")
        else:
            print("--ERROR: Must BEGIN--")


if __name__ == "__main__":
    main()

When I run this, and try to create a new item in my list using the command

set a 25

I get this error:

Traceback (most recent call last):
File "/Volumes/CON/LIFE/SimpleDatabase.py", line 81, in <module>
    main()
File "/Volumes/CON/LIFE/SimpleDatabase.py", line 65, in main
    newdb.Set(part[1],part[2])
File "/Volumes/CON/LIFE/SimpleDatabase.py", line 27, in Set
    newitem = item() #Create a new one and append it
UnboundLocalError: local variable 'item' referenced before assignment

Any help would be much appreciated. I'm pretty new to Python

Answer 1

You have a few issues with your code:

1. You are shadowing the class item with a local variable of the same name.
2. You are using varlist instead of self.varlist.
3. Some of your class methods doesn't recieve a self first argument.
4. Also there is a strong convention in python to name classes with a first capital letter.

Answer 2

The line

for item in varlist: 

shadows the class item with a local variable. So that when you get to your item() it thinks you are trying to call the local variable instead of the class. You can tell that your class item is never being constructed because it would fail as you are passing no parameters to the __init__

Also you should really call your class Item. Once I did that I got the constructor error as expected.