Python

1. 记录Python中遇到的问题，方便之后查询，假如这种方式比Google快捷，便是赚到了

2. Most pythonic way to delete a file which may not exist

   try:
       os.remove(filename)
   except OSError:
       pass

1. format a floating number 尤其是不要出现可恶的科学计数法(编程中很难找到一种场景需要科学记数法)
   "{:10.4f}".format(x)
   
   or
   
   '% 6.2f' % v

2. 处理日期

   time_test = dt.strptime(str_test, '%Y-%m-%d')
   str_out = dt.strftime(time_test, '%Y-%m-%d')

计算日期之间的差

(last_date - first_date).days
(last_date - first_date).seconds
可以根据这两项算小时的差

1. pandas
   • Converting strings to floats

     df.a = df.a.astype(float).fillna(0.0) or  
     df.a = pd.to_numeric(df.a, errors='ignore')

• convert some columns into rows

  df2 = pd.melt(df, id_vars=["location", "name"], var_name="Date", value_name="Value")

• set_index

• apply 是非常重要的神器，例如这个例子

     df['channel'] = df['wise_channel'].apply(lambda x:x.split("*")[0])
     ``` 
  
  5. [most pythonic way to pad a numeric string with zeroes to the left](http://stackoverflow.com/questions/339007/nicest-way-to-pad-zeroes-to-string)  
    for strings

  i.zfill(3)

  for numbers

  ‘{0:03d}’.format(4)

    
  6. 多进程
  
  
  7. logging 消息重复问题  
      多次循环调用程序时，日志出现第一条记录打印一条，第二次记录打印两条，依次类推，导致后续大量重复的日志输出。原因是：重复调用logging，由于logger的name是一样的，每次调用都会添加handler，多个handler就会打印多条同样的日志记录。解决办法：存在handler就不重复创建了，直接返回logger。这一篇csdn的博客可以作为参考:[Python logging 重复写日志问题](http://blog.csdn.net/huilan_same/article/details/51858817)
      ```python
      logger = logging.getLogger(logname)
      if not logger.handlers:
          balabala...
      return logger

1. if path does not exist create

   if not os.path.exists(directory):
       os.makedirs(directory)

1. pythonic way to create 3d dict

   json_dict = defaultdict(lambda: defaultdict(dict))

2. 二维字典初始化

   from collections import defaultdict, Counter
   d = defaultdict(Counter)

3. pythonic way to list all files of a directory in Python

4. Display image as grayscale using matplotlib

   import numpy as np
   import matplotlib.pyplot as plt
   from PIL import Image
   
   fname = 'image.png'
   image = Image.open(fname).convert("L")
   arr = np.asarray(image)
   plt.imshow(arr, cmap='gray')
   plt.show()

1. python的缩进是天使也是恶魔，尤其是使用vim的时候

2. 不遵守pep8的都应该受惩罚

15.