[FIXED] Python deque maxlen does not show

Issue

I have

from collections import deque

dq = deque(range(10), maxlen =  10)
dq

dq.extendleft([10, 20, 30, 40])
dq

result

deque([40, 30, 20, 10, 3, 4, 5, 6, 7, 8])

but in book Fluent Python (2019), I see maxlen, like this

deque([40, 30, 20, 10, 3, 4, 5, 6, 7, 8], maxlen=10)

Is the different cause by version different?

Solution

This seems to be caused by IPython. If you do print(dq) or print(repr(dq)), you get your expected output, and same in a normal REPL.

In [1]: from collections import deque

In [2]: dq = deque(range(10), maxlen=10)

In [3]: dq
Out[3]: deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

In [4]: print(dq)
deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], maxlen=10)

In [5]: print(repr(dq))
deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], maxlen=10)

>>> from collections import deque
>>> dq = deque(range(10), maxlen=10)
>>> dq
deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], maxlen=10)

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Update: I've submitted a PR to IPython to fix the problem.

Answered By - wjandrea