Initialise a list to a specific length in Python [duplicate]

Question

How do I initialise a list with 10 times a default value in Python?

I'm searching for a good-looking way to initialize a empty list with a specific range. So make a list that contains 10 zeros or something to be sure that my list has a specific length.

Alex Martelli · Accepted Answer · 2009-06-11 21:14:02Z

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If the "default value" you want is immutable, @eduffy's suggestion, e.g. [0]*10, is good enough.

But if you want, say, a list of ten dicts, do not use [{}]*10 -- that would give you a list with the same initially-empty dict ten times, not ten distinct ones. Rather, use [{} for i in range(10)] or similar constructs, to construct ten separate dicts to make up your list.

answered Jun 11, 2009 at 21:14

Alex Martelli

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3 Comments

Martin Konecny Over a year ago

or [{} for _ in range(10)] to avoid lint warnings

John Wu Over a year ago

something like [0] * 10 is not the way to create 2d list. If you create a list l = [[0]*10]*10], try change l[0][0] = 100, you will find that l[1][0], l[2][0] ... l[9][0] are all set to 100. It is because * replicates reference for object.

RobotHumans Over a year ago

Just hit that one. Wound up with [['' for i in range(5)] for j in range(5)] instead of >>> card_strings = [['']*5]*5 >>> card_strings[0][0] = "Well that was unexpected..."

eduffy · Accepted Answer · 2009-06-11 21:10:56Z

192

list multiplication works.

>>> [0] * 10
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

answered Jun 11, 2009 at 21:10

eduffy

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1 Comment

illan Over a year ago

WARNING: This is very prone to error due to pass by reference: self.seen = [[False] * len(board[0])] * len(board) self.seen[row][column] = True # Sets entire column, not one cell

xoxox · Accepted Answer · 2019-01-10 10:05:13Z

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In a talk about core containers internals in Python at PyCon 2012, Raymond Hettinger is suggesting to use [None] * n to pre-allocate the length you want.

Slides available as PPT or via Google

The whole slide deck is quite interesting. The presentation is available on YouTube, but it doesn't add much to the slides.

edited Jan 10, 2019 at 10:05

xoxox

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answered May 7, 2012 at 5:24

Pejvan

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