Newbie: is a variable defined?

[Nicodemus]

Christopher Swingley wrote:

>Greetings,
>
>
>Longer version: What I'm trying to do is parse a file that may or may 
>not contain an email address.  If it does, I use regular expressions to 
>extract the username portion of the email address and place this in a 
>variable named 'efrom'.  Later, I want to search a SQL database, and if 
>'efrom' has been defined it will perform one SELECT and if it's not 
>it'll do something else.
>
>I could set up a seperate flag 'efrom_defined = 0', update it when efrom 
>gets a value, and then test this flag.  Or I could use a 'try: except 
>NameError:' block.  Or I could do 'efrom = ""' at the beginning of the 
>program and then test 'if len(efrom):'
>
>How would you do this?
>

Hail!

You can do what you want by checking if the variable has an entry in the 
locals dir:

 >>> 'efrom' in locals()
0
 >>> efrom = 10
 >>> 'efrom' in locals()
1

But I would recommend against that.

The standard way is to initialize all the variables with None, and then 
check later if they were
initialized with something else.

 >>> efrom = None
 >>> efrom = 'xx at xx.com'
# later...
 >>> if efrom is not None:
...    # use 'efrom' variable here



Farewell,
Nicodemus.