Python TypeError: string indices must be integers when script runs as stand alone

Question

I wrote a script that performs some operations on a MongoDB database. Like inserts, queries, listing dbs and collections.

The function I created that selects a database works fine if I run it from another script. But crashes with this error if I run the script stand alone:

Traceback (most recent call last):
  File ".\ec2_mongo.py", line 212, in <module>
    main()
  File ".\ec2_mongo.py", line 194, in main
    mongo_select_all()
  File ".\ec2_mongo.py", line 113, in mongo_select_all
    mydb, instance_col = set_db()
  File ".\ec2_mongo.py", line 62, in set_db
    instance_col = mydb[instance_col]
TypeError: string indices must be integers

What confuses me is that the lines that works when the script is called from another one, crashes when the script runs on it's own.

These are the lines in question:

instance_col = 'ec2_list-' + today
instance_col = mydb[instance_col]

Here is the function I wrote that only crashes when the script is run on it's own:

def connect_db():
    try:
        myclient = MongoClient(
                host = "mongodb://localhost:27017/",
                serverSelectionTimeoutMS = 3000 # 3 second timeout
            )
    except errors.ServerSelectionTimeoutError as e:
        # set the client instance to 'None' if exception
        myclient = None
        # catch pymongo.errors.ServerSelectionTimeoutError
        print ("pymongo ERROR:", e)
    return myclient

def set_db():
    myclient = connect_db()
    today = datetime.today()
    today = today.strftime("%m-%d-%Y")
    if __name__ == '__main__':
        message = "Select Database"
        banner(message)
        print(Fore.CYAN + "Available MongoDB Databases:")
        if myclient != None:
            # the list_database_names() method returns a list of strings
            database_names = myclient.list_database_names()
            counter = 1
            for db in database_names:
                message = str(counter) + '. ' + db
                print(message)
                counter = counter + 1
        print ("There are", len(database_names), "databases.\n")
        print(f"Please select a database. Enter a number 1 through {len(database_names)}.")
        choice = input("Enter a number: ")
        if is_digit(choice) == True:
            if int(choice) > counter:
                print("Wrong selection.")
                set_db()
            choice = int(choice)
            choice = choice - 1
            mydb = database_names[choice]
            print(f"You've selected: {mydb}\n")
        else:
            print("Must enter a digit. Try again.\n")
            set_db()
        ## Run as a stand alone script
        instance_col = 'ec2_list-' + today
        print(f"***Instance col type: {type(instance_col)}***")
        instance_col = mydb[instance_col]
        print(f"Type MYDB: {mydb}")
        time.sleep(10)
    else:
        # Called from
        mydb = myclient['aws_ec2_list']
        print(f"Type MYDB: {mydb}")
        time.sleep(10)
        instance_col = 'ec2_list-' + today
        print(f"***Instance col type: {type(instance_col)}***")
        time.sleep(30)
        instance_col = mydb[instance_col]
    return mydb, instance_col

Obviously, `mydb` turned into a `string` instead of a `dict`. — Panwen Wang, Jul 15 '20 at 21:29

Panwen Wang · Accepted Answer · 2020-07-16T17:12:17.240

2

The reason @npwhite listed is correct.

Probably a quick fix:

Just change:

mydb = database_names[choice]

to:

mydb = myclient[database_names[choice]]

edited Jul 16 '20 at 17:12

answered Jul 15 '20 at 21:49

Panwen Wang

3,573
1
18
39

Thanks. If I try that I get an error that says: `TypeError: name must be an instance of str` – bluethundr Jul 15 '20 at 23:25
This is the full traceback: https://pastebin.com/XKQ5UHL5 – bluethundr Jul 15 '20 at 23:27
This is my function as it looks now: https://pastebin.com/9izDa1fd – bluethundr Jul 15 '20 at 23:28
Any chance to print `mydb` out after `mydb = myclient[choice]`, I doubt it is `bytes` instead of `str`. – Panwen Wang Jul 15 '20 at 23:29
Is `aws_ec2_list` in your `database_names`? – Panwen Wang Jul 15 '20 at 23:31
The `mydb` type is ` – bluethundr Jul 16 '20 at 01:12
So theoretically, if you do `mydb = myclient['aws_ec2_list']` instead of `mydb = myclient[choice]`, it should work. Could you try that as well? – Panwen Wang Jul 16 '20 at 15:59
Yeah that would But the whole point of this behavior is that if the script is run directly the user can choose a database from a menu. But if you run it as an imported module the database you are using is pre-determined. – bluethundr Jul 16 '20 at 16:43
I know, but we are debugging to see where the problem is. We are gonna finally go back to `choice`. – Panwen Wang Jul 16 '20 at 16:45
OK sure. let me try that now. Thx. – bluethundr Jul 16 '20 at 16:46
Then what happens when you enter `'aws_ec2_list'` as `choice`? (Remember change the code back to `mydb = myclient[choice]` – Panwen Wang Jul 16 '20 at 16:52
I'll try that now. The script as we had it works both on import and when you run it directly. – bluethundr Jul 16 '20 at 16:58
Yes that works. The script works both when you run it directly and on import when it's in this state: https://pastebin.com/Z3n80DBM But I have not yet added in the `if __name__ == '__main__':` section back yet. The purpose of that is to suppress the database choice menu if you are running it on import. – bluethundr Jul 16 '20 at 17:03
So it works when you entered `'aws_ec2_list'` as `choice`, but fails when you entered another database name? – Panwen Wang Jul 16 '20 at 17:10
If I do reintroduce `if __name__ == '__main__':` into the script, we are back at square 1: https://pastebin.com/aqazRURT – bluethundr Jul 16 '20 at 17:10
Could you try my updated version? Haven't noticed `choice` was a number. – Panwen Wang Jul 16 '20 at 17:13
Yes that works perfectly! Thank you for all the help! – bluethundr Jul 16 '20 at 17:26
Cool! We finally made it! – Panwen Wang Jul 16 '20 at 17:28

npwhite · Answer 2 · 2020-07-15T22:19:38.147

1

The reason the behaviour is different depending on whether you run the script directly or import it is because of the if __name__ == '__main__': condition. When you run the module directly, __name__ will be equal to "__main__", however if the module is imported, __name__ will not equal "__main__". This answer explains in more detail.

The exception is being thrown because mydb has type str, and you are trying to index the string with another string which is invalid.

In the if __name__ == '__main__' branch, mydb is a string since its assigned the result of database_names[choice], where database_names is a list of strings, so mydb will be a string too.

In the else branch, mydb is assigned the result of myclient['aws_ec2_list'], which is the dictionary-style way of getting a database instance from a mongo client instance shown here, so mydb will be a database instance.

edited Jul 15 '20 at 22:19

answered Jul 15 '20 at 21:47

npwhite

36
4

OK yes, thanks I've got that about the `if __name__ == '__main__:` condition already. What I want to know is why `mydb` becomes a string when run under `__main__` and it a mongodb object when the script is imported. How would I correct that? – bluethundr Jul 15 '20 at 21:50
@bluethundr My edit might clear things up a bit more. – npwhite Jul 15 '20 at 22:22
why does `mydb` only become a `str` type if I use it in the `if __name__ == '__main__':` block? That only happened when I introduced that if/then statement. If I remove it, and run it the way I did before I introduced the if/then it works. Real curious why that's happening. – bluethundr Jul 16 '20 at 11:02
This version of my script works whether it's a stand alone or on import. https://pastebin.com/nXeS4LSu It's when I introduce the `if __name__ == '__main__':` statement that it blows up! – bluethundr Jul 16 '20 at 11:48
The pastbin version works because you assign mydb twice, the first as a string and the second as the db instance. Since the db instance overwrites the `str`, if effectively cancels out your error. `mydb` is becoming a `str` because you are assigning it to be a string, `database_names[choice]` will yield a `str`, `myclient['aws_ec2_list']` will yield a db instance. – npwhite Jul 16 '20 at 20:42
OK thanks for that explanation. I was able to fix this by using this statement to do the assignment: `mydb = myclient[database_names[choice]]` – bluethundr Jul 16 '20 at 20:51

Python TypeError: string indices must be integers when script runs as stand alone

2 Answers2