Python parsing the lines from a file

Question

New to python. I'm reading from file line by line:

with open("graph.txt", "r") as f:
 comList = f.readlines()
 print(comList)
 edge_u = [x[0] for x in comList]
 edge_v = [x[1] for x in comList]

graph.txt has :

[(0, 7), (1, 9), (1, 9), (2, 0)]

[(2, 1), (2, 1), (3, 6)]

I was expecting that readlines will parse the file line by line hence will parse the text as a list of list. But its parsing it as List of strings. Hence i;m unable to perform the other two operations. Tried to print edge_u and get to know what its parsing. How to deal with this? Thanks.

Answer 1

You may try this:

import ast
with open("test.txt", "r") as f:
 for line in f:
 li = ast.literal_eval(line)
 edge_u = [x[0] for x in li]
 edge_v = [x[1] for x in li]

• pythad: @user2754673, You don't need to close the file if you open it with with.
• user2754673: Worked Like a charm,, !! Thanks.
• Mark Tolonen: for line in f.readlines() is redundant. for line if f is all you need.
• Mark Tolonen: @user2754673, the open file object f can be interated directly, yes.
• Mark Tolonen: Upvoted for ast.literal_eval instead of direct eval being used in other answers. It's safer.

Answer 2

file.readlines([size]) returns size bytes worth of lines or all lines as a list of strings. I believe what you want if you want to parse this file would be json.

from json import loads
with open('graph.txt', 'r') as fob:
 comList = [loads(line) for line in fob]
edge_u = [x[0] for x in comList]
edge_v = [x[1] for x in comList]

• Pynchia: your code isn't working with the given input. ValueError: Expecting value: line 1 column 2 (char 1)
• Pynchia: same result unfortunately
• OnGle: Ahh it must be the the parenthesis throwing json off. Sorry I couldn't help.

Answer 3

You could use eval but be careful not to catch something exceptional:

In [1]: tmp = eval("[(0, 7), (1, 9), (1, 9), (2, 0)]")
In [2]: tmp
Out[2]: [(0, 7), (1, 9), (1, 9), (2, 0)]

Otherwise, you have to manually parse the string (what is actually the better solution).

Answer 4

As suggested already, you can use eval

with open("graph.txt","r") as f:
 lines = f.readlines()
 comList = list(map(eval, lines))
 print(type(comList), comList)
 edge_u = [x[0] for x in comList]
 edge_v = [x[1] for x in comList]
print(type(edge_u), edge_u)
print(type(edge_v), edge_v)

which produces

<class 'list'> [[(0, 7), (1, 9), (1, 9), (2, 0)], [(0, 7), (1, 9), (1, 9), (2, 0)]]
<class 'list'> [(0, 7), (0, 7)]
<class 'list'> [(1, 9), (1, 9)]