String control with for-loop and dict in Python

Question

I wrote a dict:

msgs = {"msg_1" : "a", "msg_2" : "b", "msg_3" : "c"}

now I don't want to read in the dict manually, because I don't know how many elements will be in there (I read the dict from a file):

z = "msg_{j}"
for i in range(1, len(msgs) - 1):
    y = z.format(j=i) # This makes it "msg_1", "msg_2", "msg_3", ...
    print(msgs[y])

This throws a KeyError: msg_1. If I print out one-by-one it works just fine:

print(msgs["msg_1"])
print(msgs["msg_2"])
print(msgs["msg_3"])

... a
... b
... c

Any idea what the reason is? Why is this the case?

I tested all the functions and everything work just fine, until I get to the part with the loop (and if I use print() instead of the loop it works fine).

The code you have shown seems to have resolved the issue you have in your actual code. On another note, `range(1, len(msgs)-1)` would only only iterate through 1 item - because range stops at `n-1`, So you need `range(1, len(msgs)+1)` — karthikr, Dec 02 '16 at 15:42

score 1 · Accepted Answer · edited May 23 '17 at 12:08

1

You'd want to iterate over your dictionary.

For Python 2.x:

for key, value in msgs.iteritems():

For Python 3.x:

for key, value in msgs.items():

More details can be found here.

edited May 23 '17 at 12:08

Community

1
1

answered Dec 02 '16 at 15:40

SarTheFirst

350
1
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score 1 · Answer 2 · answered Dec 02 '16 at 15:47

In your case, replace len(msgs) - 1 with len(msgs) + 1,

msgs = {"msg_1" : "a", "msg_2" : "b", "msg_3" : "c"}

z = "msg_{j}"
for i in range(1, len(msgs) + 1):
    y = z.format(j=i) # This makes it "msg_1", "msg_2", "msg_3", ...
    print(msgs[y])


# Output
a
b
c

score 1 · Answer 3 · answered Dec 02 '16 at 16:04

1

I think your are missing the .keys() method of dict object. It returns a list of all keys in the dict object.

for key in msgs.keys():
    print msgs[key]

answered Dec 02 '16 at 16:04

sid-m

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score 0 · Answer 4 · answered Dec 02 '16 at 15:49

0

Check out the solution from SarTheFirst. If you want to print in sorted order:

for key in sorted(msg):
    print(msgs[key])

answered Dec 02 '16 at 15:49

Hai Vu

37,849
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This may not be sorted the way they want `msg_1`, `msg_10`, `msg_100`, `msg_11`, `msg_12`, ... if they have 100 items. – Duncan Dec 02 '16 at 15:54

score 0 · Answer 5 · answered Dec 02 '16 at 16:09

0

All of you thanks, I found the fix for exactly my problem.

string = "msg_{j}"

does not work, no idea why. BUT what works, is that you use the r"". So string = r"msg_{j}" works, for some reason.

answered Dec 02 '16 at 16:09

J.Horr

117
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String control with for-loop and dict in Python

5 Answers5