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looking for lists

Check a list for a list

Each time you process an item in your list, you need to check to see if the
item is another list. If the item is a list, you need to process the nested list
before processing the next item in your outer list. Deciding what to do when in
Python follows the familiar if... else... pattern:
The keyword “if” A colon (:) follows your
indicates the start condition test.
of the decision code.
if :
some condition holds

This code executes if the the "true" suite
condition holds (i.e., it’s TRUE).

else: Look! Another colon. Note: both suites
are indented.

This code executes if the condition the "false" suite
does NOT hold (i.e., it’s FALSE).

No surprises here, as the if statement in Python works pretty much as
expected. But what condition do you need to check? You need a way to
determine if the item currently being processed is a list. Luckily, Python ships
with a BIF that can help here: isinstance().
What’s cool about the isinstance() BIF is that it lets you check if a
specific identifier holds data of a specific type:

Create a short list and Let’s use the IDLE shell to learn a little about how isinstance() works:
assign it to an identifier.
>>> names = ['Michael', 'Terry']
Ask if “names” is a list (it is). >>> isinstance(names, list) Refer to a Python type
Assign a number to an True here. In this case, the type
>>> num_names = len(names)
identifier. >>> isinstance(num_names, list) is “list”.
Ask if “num_names” is a False
list (it isn’t).

20 Chapter 1