How to use logger to print a list in just one line in Python

Question

I want to print a list using logging in only one line in Python 3.6. Currently my code looks like this.

logger = logging.getLogger()
logger.setLevel(log_level)
ch = logging.StreamHandler(sys.stdout)
ch.setLevel(log_level)
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
ch.setFormatter(formatter)
logger.addHandler(ch)

# some codes in-between

num_list = [1, 2, 3, 4, 5]
logger.info("Numbers in num_list are: ")
for item in num_list:
    logger.info(item)

What I want to get is

2018-07-23 17:29:30,200 - root - INFO - Numbers in num_list are: 1 2 3 4 5

However, I would get

2018-07-23 17:29:30,200 - root - INFO - Numbers in num_list are:
2018-07-23 17:29:30,200 - root - INFO - 1
2018-07-23 17:29:30,200 - root - INFO - 2
2018-07-23 17:29:30,200 - root - INFO - 3
2018-07-23 17:29:30,200 - root - INFO - 4
2018-07-23 17:29:30,200 - root - INFO - 5

I know if I used print to output, I could have use print(item, end=" ") to explicitly change what follows the output. However, it seems that the logging won't accept end as an input parameter.

Does anyone have any ideas how I could get the desired output? Many thanks!

Solution 1

You are using a for loop which iterates over all of your list and logs it one by one try: logger.info("Numbers in num_list are: {}".format(' '.join(map(str, num_list)))) to post them all in once

See: https://docs.python.org/3/library/stdtypes.html?highlight=str#str.join

Solution 2

There is a strong difference between an output stream (used by print) and a message log. The stream is a sequence of characters (or bytes for binary strings) that may happen to contain newline characters. Those newlines characters are then interpreted when you display them on a terminal (or when you print them).

A log is a sequence of messages, and each message is supposed to be atomic. Once a message has been logged, you cannot add anything to it but can only log new messages.

So you have to first fully build your message and then log it:

num_list = [1, 2, 3, 4, 5]
msg = "Numbers in num_list are: " + " ".join(num_list)      # fully build the message
logger.info(msg)                                            #  and then log it

Solution 3

Not exactly what you want, but more lazy: It may be usefull when you want to create some fast debug:

num_list = [1, 2, 3, 4, 5]
logger.info(str(("Numbers in num_list are: ",num_list))

output:

('Numbers in num_list are: ', [1, 2, 3, 4, 5])

Solution 4

Another nice short way I've found here -

nums = [3, 87, 28, 25, 96]
logging.debug("Here are my favourite numbers: " + str(nums)[1:-1])

"""
 Output:
 Here are my favorite numbers: 3, 87, 28, 25, 96
""" 

One more using map -

logging.debug("Favourite numbers are: ".join(map(str, nums))

View more solutions

Comments

[kabanus]

Why aren't you passing the list? as in logger.info(num_list)?

[Jekyll SONG]

Thanks for the insightful comments. I guess the lesson here is that I try too hard on finding the connection between logging and print to forget that I could actually concatenate the string before output it. I have been trying to change the logging.config but ignore the simpler way

[Maico Timmerman]

It is better to let the logging module do the actual formatting using the %-syntax, since the message might not be printed at all.

[idbrii]

This deferred execution is a neat idea. The implementation could be very simple but still recursive by using pprint.pformat(self._seq) to build the string. pprint is part of the standard library. However, it looks like logging already supports deferred formatting.

[idbrii]

I guess this would be a good solution if you wanted to customize how to print the data (non default args to pformat).

[idbrii]

@MaicoTimmerman: Is using %s like in my answer what you're referring to?

[gladd]

@idbrii Gads! I don't know how I missed that Python has supported the string formatting operator for ages. I must have been thinking Java6 when I wrote it. Been away from Python for a while :-S

[gladd]

Thanks for the kind words of support re customising @idbrii, however it was me just being dufus!