How do I get the value of a btc address transaction with pybitcoin tools?

Lets say that I execute the code below when the user accesses the deposit page.

my_private_key = random_key()
my_public_key = privtopub(my_private_key)
my_address = pubtoaddr(my_public_key)

And I let the user deposit into my_address.

How would I obtain the value of the users deposit into my_address?

You can use the unspent command to find unspent outputs sent to that address, or history to see all outputs to that address whether they are spent or not. Note that it relies on public APIs to read the blockchain for transactions.