python: creating a list inside a dictionary

Question

I just started using python and I'm trying to create a program that will read a file that looks like this:

AAA x 111
AAB x 111
AAA x 112
AAC x 123
...

the file is 50 lines long and I'm trying to make the letters into keys in a dictionary and the numbers lists that correspond with the keys.

I want the output to look like this:

{AAA: ['111', '112'], AAB: ['111'], AAC: [123], ...}

This is what I've tried

file = open("filename.txt", "r") 
readline = file.readline().rstrip()
while readline!= "":
    list = []
    list = readline.split(" ")
    j = list.index("x")
    k = list[0:j]
    v = list[j + 1:]
    d = {}
    if k not in d == False:
        d[k] = []
    d[k].append(v)
    readline = file.readline().rstrip()

I keep getting syntax errors on my if statement and I can't figure out what I've done wrong.

Solution 1

You're missing colons at the ends, also you could write this like:

if k not in d:
    d[k] = [v]
else:
    d[k].append(v)

Solution 2

Makes your life a little easier with defaultdict

>>> out = defaultdict(list)
>>> with open('test.txt') as f:
      for line in f.readlines():
        x = line.split()
        out[x[0]].append(int(x[2]))

It's also good practice to use the with statement to open files, when possible. Your original example had two entries as str, and one as int, I have just converted all to int here.

Solution 3

You need colons at the ends of your if statements; this version should work:

   if k in d == False:
      d[k] = []
      d[k].append(v)
   else:
      d[k].append(v)

Solution 4

Sounds like a job for defaultdict! Aside from your missing colon, as mentioned by others, you could replace the whole if with this:

from collections import defaultdict # placed with your other imports, probably
....
d = defaultdict(lambda:[])
d[k].append(v)

View more solutions

Comments

[Nathan]

You could consolidate list = [] and list = readline.split(" ") into just list = readline.split(" ")- by redefining list, you're making the previous definition disappear. Also, str.split returns a list.

[Nathan]

Also, try to stay away from using list as a variable name - list is a builtin.

[Aesthete]

@kuyan - I think your second example is supposed to be list = readline.split()?

[Nathan]

@Aesthete: You're right, I wasn't thinking.

[Gareth Latty]

This is bad in a few ways. Firstly, if k not in d: would read better, you repeat d[k].append(v) instead of moving it outside the if, and you introduce a race condition into your code by not trying then catching the exception, but rather by checking beforehand. It's not likely to be a problem, but it's still a bad idea.

[Aesthete]

The argument to the defaultdict constructor must be callable.

[Keenan]

I tried replacing the if statement with the defaultdict but I get a TypeError: first argument must be callable

[Keenan]

I followed your comments and cleaned up the code slightly. Now when the program executes I get TypeError: unhashable type: 'list'

[acjay]

Sorry! My fault for not testing. Now using a lambda to make a constructor that returns the empty list. Aesthete's solution with the list constructor is probably better, but I'll leave it like this as another solution.