Pitfalls in Python

Mark the pitfalls I fell into while writing Python.

#1

>>> {1: 1, 1: 2}  # expect error
{1: 2}

#2

You should always add # coding: utf-8 to the head of each Python source file, especially when writing production code.

#3

List multiplying is error-prone.

>>> matrix = [[0] * 4] * 4
>>> matrix
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
>>> matrix[0][0] = 1
>>> matrix
[[1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0]]  # surprise!!!