Python’s datetime module provides powerful tools for working with dates and times. Often, you’ll need to format these datetime objects into strings, and a common requirement is to include milliseconds in the output. This tutorial explains how to achieve this using different approaches.
Understanding the Basics
The core of formatting datetime objects lies in the strftime() method. This method takes a format string as an argument, specifying how the date and time should be represented in the resulting string. Several format codes are available to represent different parts of the date and time.
Using strftime() with Microseconds and Truncation
The %f format code represents microseconds (six digits). If you only need milliseconds (three digits), you can truncate the last three digits from the microseconds string.
from datetime import datetime
now = datetime.now()
formatted_datetime = now.strftime('%Y-%m-%d %H:%M:%S.%f')[:-3]
print(formatted_datetime)
This code first formats the datetime object to include microseconds, then slices the string to keep only the first three digits after the decimal point, effectively representing milliseconds.
Using isoformat() with timespec (Python 3.6+)
Python 3.6 introduced the timespec argument to the isoformat() method. This provides a more direct way to specify the desired precision (in this case, milliseconds).
from datetime import datetime
now = datetime.now()
formatted_datetime = now.isoformat(sep=' ', timespec='milliseconds')
print(formatted_datetime)
This approach is cleaner and more readable than string slicing, as it directly specifies the desired level of precision.
Handling Potential Issues with Older Systems (Python 2.7)
On some older systems, particularly Python 2.7 on Windows, the %f format code might not behave as expected, potentially returning "0" instead of the actual microseconds. If you encounter this issue, you can explicitly split the string and perform the truncation:
from datetime import datetime
now = datetime.now()
dt_str, micro_str = now.strftime('%Y-%m-%d %H:%M:%S.%f').split('.')
millisecond_str = "%s.%03d" % (dt_str, int(micro_str) // 1000)
print(millisecond_str)
This approach provides a more robust solution for systems where %f does not reliably return the full microsecond value.
Choosing the Right Method
- For Python 3.6 and later, using
isoformat(timespec='milliseconds')is the recommended approach due to its clarity and directness. - For older Python versions, the
strftime()with truncation method is a reliable and portable solution. - If you are concerned about compatibility with potentially problematic systems (like Python 2.7 on Windows), the explicit splitting and truncation method is the most robust option.
By understanding these different approaches, you can confidently format datetime objects with milliseconds in Python, ensuring compatibility and accuracy across various environments.