Maybe the default value here should be None instead of api=default_api that'll make supporting the parameter easier for wrappers of things which take APIs?  You want to encourage wrappers to support passing an API implementation through them and it's easier to do that now.  (I'm assuming making this a required arg is off the table already.)

```python
def someCryptoThingy(params, blah, api=None):
   …
   block = BlockCipher(cipher, mode, api)
```

seems better than

```python
from cryptography.bindings import default_api

def someCryptoThingy(params, blah, api=default_api):
   …
   block = BlockCipher(cipher, mode, api)
```