Convert and merge strings into a list in Python

Question

In Python I have four strings that include the formatting of a list:

line1 ="['a.b.c','b.c.a','c.d.e']"
line2 ="['def','efg']"
line3 ="['f']"
line4 ="['g']"

How do I merge them all so I get a valid Python list such as:

SumLine = ['a.b.c','b.c.a','c.d.e','def','efg','f','g']

Answer 1

import ast

line1 ="['a.b.c','b.c.a','c.d.e']"
line2 ="['def','efg']"
line3 ="['f']"
line4 ="['g']"

SumLine = []

for x in (line1, line2, line3, line4):
  SumLine.extend(ast.literal_eval(x))

print SumLine

Don't use the built-in eval unless you have preternatural trust in the strings you're evaluating; ast.literal_eval, while limited to simple constants, is totally safe and therefore, most often, way preferable.

Answer 2

Try eval:

>>> line1 ="['a.b.c','b.c.a','c.d.e']"
>>> line2 ="['def','efg']"
>>> line3 ="['f']"
>>> line4 ="['g']"
>>> eval(line1) + eval(line2) + eval(line3) + eval(line4)
['a.b.c', 'b.c.a', 'c.d.e', 'def', 'efg', 'f', 'g']

But be careful, because eval can be dangerous. Don't use it on input that you receive from the user and haven't validated.

Answer 3

you need to eval them first and then you could sum the results. But I wonder how do you get this strings in the first place?

Answer 4

The quick and dirty way is to use eval:

SumLine = eval(line1) + eval(line2) + eval(line3) + eval(line4)

But dont do this if you are getting these strings from someone else (ie user input)

Answer 5

The simple way is to concetenate the strings to an expression that can be evaulated to give the required result:

line1 ="['a.b.c','b.c.a','c.d.e']"
line2 ="['def','efg']"
line3 ="['f']"
line4 ="['g']"
lines = [line1, line2, line3, line4]

print eval('+'.join(lines))

However this is unsafe if you can't trust your input, so if you're using Python 2.6 or higher you should use the safe eval function ast.literal_eval in the ast module, although this doesn't work with the '+' trick so you will have to iterate over each element instead.

Answer 6

Where did you get these strings? Anything short of a real parser will be fragile. Below is what I would recommed, if I had not seen Alex Martelli's brilliant answer before!

You may parse them as JSON arrays, but JSON wants to read double-quoted strings, not single quotes. This introduces fragility to the method, but still much preferable to eval() which is unsafe.

import json
line1 ="['a.b.c','b.c.a','c.d.e']"
json.loads(line1.replace("'", '"'))

The result is a parsed list like [u'a.b.c', u'b.c.a', u'c.d.e'], you may than go on to join the parsed lists.