Reading a list of lists into Python

Question

I have made a list of lists to store score data for a given student. The list is called "class1" and each student has a name followed by 3 scores. I can sort this list fine and can write to a file which show the data in the correct format:

class1=[["Tom",7,2,1],["Jo",8,0,0],["Adelphe",9,0,0]]

When written look like this which I am happy with:

['Tom', 7, 2, 1]
['Jo', 8, 0, 0]
['Adelphe', 9, 0, 0]

The problem comes when I try to read the same list in. Each sublist is read as a string. I can't seem to change this.

Here is the code:

class1 = [line.strip() for line in open("data2.txt", 'r')]

This is what the list looks like when read, as you can see there are apostrophes around each sub list, meaning they are strings within the class1 list:

["['Tom', 7, 2, 1]", "['Jo', 8, 0, 0]", "['Adelphe', 9, 0, 0]"]

Can anyone help me, I've done a lot of searching, but can't find anyone with the same problem.

Thanks, Tom

Why not use JSON or CSV instead of inventing your own format? — Wooble
– Wooble, Commented Oct 27, 2014 at 11:33
Try using eval(string). If the string represents a list, it should become a python list object. I.e. class1 = [eval(line.strip()) for line in open("data2.txt", 'r')] — SomethingSomething
– SomethingSomething, Commented Oct 27, 2014 at 11:33
If the file you're writing to doesn't need to be human readable/editable, you can use the pickle module to write it and load it... will be more efficient that storing as text, then evaluating the input on reading again — Jon Clements
– Jon Clements, Commented Oct 27, 2014 at 11:57

Mauro Baraldi · Accepted Answer · 2014-10-27 11:53:07Z

1

>>> import ast
>>> with open('/path/to/data2.txt', 'r') as classes:
        lines = [ast.literal_eval(line.strip()) for line in classes]

>>> print lines
[['Tom', 7, 2, 1], ['Jo', 8, 0, 0], ['Adelphe', 9, 0, 0]]

answered Oct 27, 2014 at 11:53

Mauro Baraldi

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5 Comments

Tom Smith Over a year ago

Hi, eval worked. What is the benefit of using: ast.literal_eval? Pardon my ignorance!

anon582847382 Over a year ago

@TomSmith ast.literal_eval protects against arbitrary code execution.

wwii Over a year ago

@TomSmith eval will evaluate any valid expression which makes it possible for unknown data to harm your computer, like creating a huge string and eating up your processor time and/or memory. Unless you are 100% sure of your data don't use eval, ast.literal_eval is safer because it restricts the types of expressions it will eveluate.

Tom Smith Over a year ago

@wwii Note taken! Thanks

Martijn Pieters Over a year ago

@TomSmith: ast.literal_eval() only supports save Python literals as input; strings, numbers, booleans, containers. Nothing more. eval() supports arbitrary Python expressions, including everything that a process can do to damage your computer or turn it into a subservient internet spam bot. It is for that reason I voted to delete the answer you marked as accepted; it is dangerous to use eval(), and you should avoid it for everything but the most specialised of cases where you are sure not to be executing user-supplied input.

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