python - Python : SyntaxError

Question

我正在编写用于解析电子邮件的脚本，但是以下部分的for循环上有一些SyntaxError:

def main():
     writer = csv.DictWriter(open('features.csv', 'w'), list(EXTRACTS.keys()))
     for mail in os.listdir(MAILDIR):
         writer.writerow({
                key: value(email.message_from_file(open(os.path.join(MAILDIR, mail), 'r')))
            for key, value in EXTRACTS.items()
             })

请帮帮我!
编辑:

File "/IS/extraction.py", line 52

for key, value in EXTRACTS.items()
  ^ SyntaxError: invalid syntax

Answer 1

您正在尚未支持dict理解的旧Python版本上运行此代码。 {key: value for ... in ...}语法仅在Python 2.7和更高版本中可用:

Python 2.6.8 (unknown, May 22 2013, 11:58:55) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def main():
...      writer = csv.DictWriter(open('features.csv', 'w'), list(EXTRACTS.keys()))
...      for mail in os.listdir(MAILDIR):
...          writer.writerow({
...                 key: value(email.message_from_file(open(os.path.join(MAILDIR, mail), 'r')))
...             for key, value in EXTRACTS.items()
  File "<stdin>", line 6
    for key, value in EXTRACTS.items()
      ^
SyntaxError: invalid syntax

用字典构造函数和生成器表达式替换行:

writer.writerow(dict(
    (key, value(email.message_from_file(open(os.path.join(MAILDIR, mail), 'r'))))
    for key, value in EXTRACTS.items()
))

您确实希望避免阅读 EXTRACTS中每个键对对的电子邮件；每个外循环读取一次:

def main():
    writer = csv.DictWriter(open('features.csv', 'w'), list(EXTRACTS.keys()))
    for mail in os.listdir(MAILDIR):
        mail = email.message_from_file(open(os.path.join(MAILDIR, mail), 'r'))
        writer.writerow(dict((key, value(mail)) for key, value in EXTRACTS.items()))