python_使用

1、系统相关

!cmd Execute cmd in the system shell
output = !cmd args Run cmd and store the stdout in output
%alias alias_name cmd Define an alias for a system (shell) command
%bookmark Utilize IPython’s directory bookmarking system
%cd directory Change system working directory to passed directory
%pwd Return the current system working directory
%pushd directory Place current directory on stack and change to target directory
%popd Change to directory popped off the top of the stack
%dirs Return a list containing the current directory stack
%dhist Print the history of visited directories
%env Return the system environment variables as a dict
2、ipython定义类

class Message:

def __init__(self, msg):

self.msg = msg

def __repr__(self):

return 'Message: %s' % self.msg

3、类型转换

（1）rr.astype(np.int32)//类型转换

（2）string 转float

numeric_strings = np.array(['1.25', '-9.6', '42'], dtype=np.string_)
numeric_strings.astype(float)

array([ 1.25, -9.6 , 42. ])

4、array 并不copy

 (1) arr_slice = arr[5:8] //相当于引用

arr_slice[1] = 12345

Out[59]: array([ 0, 1, 2, 3, 4, 12, 12345, 12, 8, 9])

In [60]: arr_slice[:] = 64

Out[61]: array([ 0, 1, 2, 3, 4, 64, 64, 64, 8, 9])

（2）拷贝：arr[5:8].copy()

5、numpy 保存数据

(1)      numpy.save('test', var)

x = numpy.load('test.npy')

(2) 保存多个数据

numpy.savez('zdad.npz', a= arr, b= arr)

x = numpy.load('zdad.npz')

x['a'], x['b']

6、numpy 读取文件按

numpy.genfromtxt('world_alcohol.csv', delimiter = ',',dtype = 'U75', skip_header = True)、、delimiter 利用 ‘，’进行分割，u75：设置成unicode自字符，skip_header:跳过表的抬头

print (world_alcohol)

7、numpy

numpy.sum(aix=1), 每行求和

numpy.sum(aix=0), 每列求和

8、pandas 获取以 ‘g‘结尾的列

print(food_info.columns)

print(food_info.head(2))

col_names = food_info.columns.tolist()//获取表的列名

gram_columns = []

for c in col_names:

    if c.endswith("(g)"):

        gram_columns.append(c)

gram_df = food_info[gram_columns]

print(gram_df.head(3))