[python] How can I check if a key exists in a dictionary?

Let's say I have an associative array like so: {'key1': 22, 'key2': 42}.

How can I check if key1 exists in the dictionary?

Another method is has_key() (if still using Python 2.X):

>>> a={"1":"one","2":"two"}
>>> a.has_key("1")
True

If you want to retrieve the key's value if it exists, you can also use

try:
    value = a[key]
except KeyError:
    # Key is not present
    pass

If you want to retrieve a default value when the key does not exist, use value = a.get(key, default_value). If you want to set the default value at the same time in case the key does not exist, use value = a.setdefault(key, default_value).