When is a string not a string? Problems with the config method on network interfaces.

[larsks]

I'm running MicroPython v1.9.3-8-g63826ac5c on an esp8266.

Given:

>>> ap_if = network.WLAN(network.AP_IF)

This works:

>>> ap_if.config('essid')
'MicroPython-35a0d8'

And this works:

>>> key = 'essid'
>>> ap_if.config(key)
'MicroPython-35a0d8'

But this fails:

>>> key1 = 'essid'
>>> ap_if.config(key1)
'MicroPython-35a0d8'
>>> somevar = b'essid'
>>> key2 = str(somevar, 'ascii')
>>> ap_if.config(key2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: unknown config param

Even though:

>>> type(key1) == type(key2)
True
>>> key1 == key2
True
>>> repr(key1) == repr(key2)
True

What's going on here?

[larsks]

And for completeness, wrapping that final example in another call to str() works:

>>> somevar = b'essid'
>>> key2 = str(somevar, 'ascii')
>>> key3 = str(key2)
>>> ap_if.config(key3)
'MicroPython-35a0d8'

[dpgeorge]

Thanks for the report. This is a consequence of network.config() checking its incoming parameters against interned string identifiers, not actual string data. Looking into this actually opened a small can of worms, because it turns out that there are lots of places where one can create a str object that is a duplicate of an existing interned object (eg ' essid '.split()[0] will also fail in the above example).

In the interest of reducing heap allocation wherever possible, I've now fixed all the cases where new str objects could be returned without first checking if they duplicate an existing interned str. Such a fix has also fixed this issue; see 1f1d519 and 8d956c2

[larsks]

Thanks! That fixed it.