566

Just like C, you can break a long line into multiple short lines. But in Python, if I do this, there will be an indent error... Is it possible?

652

From PEP 8 - Style Guide for Python Code:

The preferred way of wrapping long lines is by using Python's implied line continuation inside parentheses, brackets and braces. If necessary, you can add an extra pair of parentheses around an expression, but sometimes using a backslash looks better. Make sure to indent the continued line appropriately.

Example of implicit line continuation:

a = some_function(
    '1' + '2' + '3' - '4')

On the topic of line-breaks around a binary operator, it goes on to say:-

For decades the recommended style was to break after binary operators. But this can hurt readability in two ways: the operators tend to get scattered across different columns on the screen, and each operator is moved away from its operand and onto the previous line.

In Python code, it is permissible to break before or after a binary operator, as long as the convention is consistent locally. For new code Knuth's style (line breaks before the operator) is suggested.

Example of explicit line continuation:

a = '1'   \
    + '2' \
    + '3' \
    - '4'
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  • 26
    For the second example, it does not like spaces (which you cannot see) after the ' \' – cardamom Mar 9 '17 at 11:49
  • @cardamom This issue is not a particularity of python; for instance, if you try to write a shell script, e.g., a for loop for creating directories: for i in dir1\ (new line) dir2\ (new line) dir3\ (new line) ... mkdir $i in order for this script to run, there must be no space after the backslash – DavidC. Aug 7 '17 at 17:26
  • @cardamom Actually it does not like any character (except newlines). Too bad for comments… – Skippy le Grand Gourou Oct 30 '19 at 10:33
222

There is more than one way to do it.

1). A long statement:

>>> def print_something():
         print 'This is a really long line,', \
               'but we can make it across multiple lines.'

2). Using parenthesis:

>>> def print_something():
        print ('Wow, this also works?',
               'I never knew!')

3). Using \ again:

>>> x = 10
>>> if x == 10 or x > 0 or \
       x < 100:
       print 'True'

Quoting PEP8:

The preferred way of wrapping long lines is by using Python's implied line continuation inside parentheses, brackets and braces. If necessary, you can add an extra pair of parentheses around an expression, but sometimes using a backslash looks better. Make sure to indent the continued line appropriately. The preferred place to break around a binary operator is after the operator, not before it.

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  • This came up during a talk on code style at PyOhio and we came to the conclusion that 2) was a good standard way to go. It's not perfect since changing the text isn't as easy, but at least it reads ok. – Rick Nov 13 '10 at 16:53
  • @user225312 could you also use''' code newline more code'''' or this a bad habit? – onxx Apr 6 '15 at 22:14
  • second one prints a tuple in python 2.7 – iggy Oct 19 '16 at 17:44
  • first one gives me a SyntaxError in IDLE(v.3.5), why? Also, if I don't start with a parenthesis gives me same error with missing parenthesis. With parentheses gives me nothing – Yannis Dran Nov 22 '16 at 23:05
  • 3
    Last sentence that mentions breaking after the operator, is opposed to what PEP8 suggests of breaking before the operator to improve readability. – milpita Jan 17 '17 at 19:02
28

If you want to assign a long str to variable, you can do it as below:

net_weights_pathname = (
    '/home/acgtyrant/BigDatas/'
    'model_configs/lenet_iter_10000.caffemodel')

Do not add any comma, or you will get a tuple which contains many strs!

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  • 7
    This works because python automatically concatenates the strings inside the parenthesis, without the need of putting a + operator. – blueFast Oct 17 '17 at 14:30
  • For string interpolation, put f before each short string, uri = (f"example.com/version8.2/apps/{self.market}" f"/app/{self.product_name}/ranks?start_date={self.week_ago_str}&end_date={self.today_str}") – Charlie 木匠 Jul 23 '18 at 19:05
  • This also works well if you place all your strings in a multi-line string, i.e. between triple ", e.g. ( """line 1 line2""") , and possibly won't carry the "comma risk" :) – jmng Jul 25 '18 at 17:17
17

It works in Python too:

>>> 1+\
      2+\
3
6
>>> (1+
          2+
 3)
6
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14

When trying to enter continuous text (say, a query) do not put commas at the end of the line or you will get a list of strings instead of one long string:

queryText= "SELECT * FROM TABLE1 AS T1"\
"JOIN TABLE2 AS T2 ON T1.SOMETHING = T2.SOMETHING"\
"JOIN TABLE3 AS T3 ON T3.SOMETHING = T2.SOMETHING"\
"WHERE SOMETHING BETWEEN <WHATEVER> AND <WHATEVER ELSE>"\
"ORDER BY WHATEVERS DESC"

kinda like that.

There is a comment like this from acgtyrant, sorry, didn't see that. :/

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  • This won't work - you need some white-space at either the beginning or end of each quoted string. There will currently be no space between "T1" and "JOIN". In the example above, just use multi-line strings with ''' or """, SQL won't mind the CR/LF's. – dsz Jan 6 at 23:16
8

As far as I know, it can be done. Python has implicit line continuation (inside parentheses, brackets, and strings) for triple-quoted strings ("""like this""")and the indentation of continuation lines is not important. For more info, you may want to read this article on lexical analysis, from python.org.

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  • 3
    Strings only do implicit continuation inside triple-quoted strings. That's the distinction between triple- and single-quoted. – Ned Batchelder Nov 13 '10 at 12:34
6

DB related code looks easier on the eyes in multiple lines, enclosed by a pair of triple quotes:

SQL = """SELECT
            id, 
            fld_1, 
            fld_2, 
            fld_3, 
            ...... 
         FROM some_tbl"""

than the following one giant long line:

SQL = "SELECT id, fld_1, fld_2, fld_3, .................................... FROM some_tbl"
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