Python: SyntaxError

Question

I'm writing a script to parse an email, but there is some SyntaxError on the for loop in the following part:

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()
             })

Please help me out of this!

EDIT:

File "/IS/extraction.py", line 52
for key, value in EXTRACTS.items()
  ^ SyntaxError: invalid syntax

value(email.message_from_file(open(os.path.join(MAILDIR, mail), 'r'))) doesnt look right. what are you trying to do? — karthikr
– karthikr, Commented Oct 18, 2013 at 19:05
@karthikr: It's fine if the values in the EXTRACTS dictionary are callable. — Martijn Pieters
– Martijn Pieters, Commented Oct 18, 2013 at 19:06
For future reference, please do include the full traceback of your exception, to make it easier for us to diagnose your problem. — Martijn Pieters
– Martijn Pieters, Commented Oct 18, 2013 at 19:07
In that line the script should read all the mails (found in the MAILDIR) and get the values we have to inspect (for example sender, IP, subject,...) — ABC
– ABC, Commented Oct 18, 2013 at 19:07

Martijn Pieters · Accepted Answer · 2013-10-18 19:11:23Z

You are running this on an older Python version that doesn't yet support dict comprehensions. The {key: value for ... in ...} syntax is only available in Python 2.7 and newer:

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

Replace the line with a dictionary constructor and a generator expression:

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

You do want to avoid reading the email message for every key-item pair in EXTRACTS though; read it once per outer loop:

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()))

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