Python 2 和 Python 3 列表推導(dǎo)的區(qū)別

測試了一個列表推導(dǎo)，在 Python 3 中無法運行，啥原因？

# python2  
matrix = [[1,2,3],[4,5,6],[7,8,9]]
[x for row in matrix for x in row]
# 值是 [1, 2, 3, 4, 5, 6, 7, 8, 9]
[x for x in row for row in matrix]
# 值是 [7, 7, 7, 8, 8, 8, 9, 9, 9]

# python3  
matrix = [[1,2,3],[4,5,6],[7,8,9]]
[x for row in matrix for x in row]
# 值是 [1, 2, 3, 4, 5, 6, 7, 8, 9]
[x for x in row for row in matrix]
# Traceback (most recent call last):
#  File "<stdin>", line 1, in <module>
# NameError: name 'row' is not defined

答：python2和python3列表推導(dǎo)的作用域有區(qū)別

在python2中先運行了[x for row in matrix for x in row] 內(nèi)存中已經(jīng)有row 因此會有結(jié)果，但是python3中優(yōu)化了這個問題，所以會報錯