在 python 中，判断字典中指定的 key 是否存在有三种方式，if key in dct、if key in dct.keys() 和 if dct.has_key(key)，其中 key in dct 形式效率最快，推荐使用。
key in dct（推荐方式）
dct = {'knowledge': 18, "dict": 8}
if 'knowledge' in dct:
print(dct['knowledge'])
key in dct.keys()
if 'knowledge' in dct.keys():
print(dct['knowledge'])
dct.has_key(key)（python 2.2 及以前）
python 2.2 及之前的版本，字典提供了查询指定 key 是否存在的函数 has_key，但在 python 3 已经删除。
if dct.has_key('knowledge'):
print(dct['knowledge'])
三种方式的效率对比
import time
from matplotlib import pyplot as plt
n = 10000
time1 = []
time2 = []
time3 = []
for n in range(100,10100,100):
my_dict = {}
for i in range(n):
my_dict[i] = i
start = time.time()
for i in range(n):
if i in my_dict.keys():
pass
end = time.time()
print end - start
time1.append(end - start)
start = time.time()
for i in range(n):
if my_dict.has_key(i):
pass
end = time.time()
print end - start
time2.append(end - start)
start = time.time()
for i in range(n):
if i in my_dict:
pass
end = time.time()
print end - start
time3.append(end - start)
t = range(100,10100,100)
plt.plot(t, time1, label='in keys()')
plt.plot(t, time2, label='has_key')
plt.plot(t, time3, label='in')
plt.legend()
plt.show()