How can I do a line break (line continuation) in Python?
I have a long line of code that I want to break up among multiple lines. What do I use and what is the syntax?
For example, adding a bunch of strings,
e = 'a' + 'b' + 'c' + 'd'
and have it in two lines like this:
e = 'a' + 'b' + 'c' + 'd'
What is the line? You can just have arguments on the next line without any problems:
a = dostuff(blahblah1, blahblah2, blahblah3, blahblah4, blahblah5, blahblah6, blahblah7)
Otherwise you can do something like this:
if a == True and \ b == False
Check the style guide for more information.
From your example line:
a = '1' + '2' + '3' + \ '4' + '5'
a = ('1' + '2' + '3' + '4' + '5')
Note that the style guide says that using the implicit continuation with parentheses is preferred, but in this particular case just adding parentheses around your expression is probably the wrong way to go.
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The preferred way of wrapping long lines is by using Python's implied line continuation inside parentheses, brackets and braces. Long lines can be broken over multiple lines by wrapping expressions in parentheses. These should be used in preference to using a backslash for line continuation.
Backslashes may still be appropriate at times. For example, long, multiple with-statements cannot use implicit continuation, so backslashes are acceptable:
with open('/path/to/some/file/you/want/to/read') as file_1, \ open('/path/to/some/file/being/written', 'w') as file_2: file_2.write(file_1.read())
Another such case is with assert statements.
Make sure to indent the continued line appropriately. The preferred place to break around a binary operator is after the operator, not before it. Some examples:
class Rectangle(Blob): def __init__(self, width, height, color='black', emphasis=None, highlight=0): if (width == 0 and height == 0 and color == 'red' and emphasis == 'strong' or highlight > 100): raise ValueError("sorry, you lose") if width == 0 and height == 0 and (color == 'red' or emphasis is None): raise ValueError("I don't think so -- values are %s, %s" % (width, height)) Blob.__init__(self, width, height, color, emphasis, highlight)
PEP8 now recommends the opposite convention (for breaking at binary operations) used by mathematicians and their publishers to improve readability.
Donald Knuth's style of breaking before a binary operator aligns operators vertically, thus reducing the eye's workload when determining which items are added and subtracted.
Donald Knuth explains the traditional rule in his Computers and Typesetting series: "Although formulas within a paragraph always break after binary operations and relations, displayed formulas always break before binary operations".
Following the tradition from mathematics usually results in more readable code:
# Yes: easy to match operators with operands income = (gross_wages + taxable_interest + (dividends - qualified_dividends) - ira_deduction - student_loan_interest)
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 is suggested.
: Donald Knuth's The TeXBook, pages 195 and 196
You can break lines in between parenthesises and braces. Additionally, you can append the backslash character
\ to a line to explicitly break it:
x = (tuples_first_value, second_value) y = 1 + \ 2
From the horse's mouth: Explicit line joining
Two or more physical lines may be joined into logical lines using backslash characters (
\), as follows: when a physical line ends in a backslash that is not part of a string literal or comment, it is joined with the following forming a single logical line, deleting the backslash and the following end-of-line character. For example:
if 1900 < year < 2100 and 1 <= month <= 12 \ and 1 <= day <= 31 and 0 <= hour < 24 \ and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date return 1
A line ending in a backslash cannot carry a comment. A backslash does not continue a comment. A backslash does not continue a token except for string literals (i.e., tokens other than string literals cannot be split across physical lines using a backslash). A backslash is illegal elsewhere on a line outside a string literal.