concatenating python strings in a loop

Question

I am using enums and string.join() method to form a help string in Python:

I have the following code segment:

from enum import Enum

class Estimators(Enum):
    rsac = 1
    msac = 2

Now, I create a help string as follows:

est_help = 'Valid options are: [' + (str(i.name) + ', ' for i in Estimators) + ']'

This throws a TypeError exception as:

TypeError: cannot concatenate 'str' and 'generator' objects

I was wondering what I am doing wrong. The i.name is of string type.

(str(i.name) + ', ' for i in RobustEstimators) is a generator, so you can't add it to a string, exactly as the error message tells you. Did you mean ', '.join(...)? — jonrsharpe
– jonrsharpe, Commented Oct 1, 2015 at 13:29
Maybe you need est_help = 'Valid options are: [' + str(i.name) + ', ' for i in RobustEstimators + ']'? — Remi Guan
– Remi Guan, Commented Oct 1, 2015 at 13:33
@jonrsharpe Oops, I didn't see the + ']', I thought that it's a list comprehension. Sorry for this :P — Remi Guan
– Remi Guan, Commented Oct 1, 2015 at 13:38

Sigve Kolbeinson · Accepted Answer · 2015-10-01 14:17:50Z

The error message tells you what you are doing wrong - attempting to concatenate a string and a generator. What you want to do is to make a list using list comprehension based on the generator, then use that

est_help = 'Valid options are: [{}]'.format( ', '.join( i.name for i in Estimators))

Let's decompose this into individual steps:

Create the list [rsac,msac]: est_list = [str(i.name) for i in Estimators]
Create a string with the list elements separated by a comma 'rsac, msac': est_str = ', '.join( est_list )
Insert the string into your text template: est_help = 'Valid options are: [{}]'.format( est_str ), and get the resulting string Valid options are: [rsac, msac]'

edit: modified code incorporating suggestions from comments

est_help = 'Valid options are: [{}]'.format( ', '.join( i.name for i in Estimators ) )

You can save some line noise by using a generator expression instead of a list comprehension - just drop the [] inside the call to join. Also, carried over from the OP but still worth mentioning, i.name is already a string, so you can drop the str call for ', '.join(i.name for i in Estimators).

panda-34 · Accepted Answer · 2015-10-01 13:54:10Z

2

You can join Estimators' members:

'Valid options are: [%s]' % ', '.join(Estimators.__members__)

answered Oct 1, 2015 at 13:54

panda-34

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Comments

djangoliv · Accepted Answer · 2015-10-01 13:51:28Z

1

Solution

est_help = 'Valid options are: [' + ",".join(str(i) for i in Estimators) + ']'

answered Oct 1, 2015 at 13:51

djangoliv

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1 Comment

lvc Over a year ago

It would be helpful if you could explain for the OP (and future visitors) how/why this code works, or how to get to it from the position in the question, rather than just saying "here's the code".

SirParselot · Accepted Answer · 2015-10-01 14:38:42Z

Since none of the posts mentioned yet work for me(I always get 'type' object is not iterable, @lvc figured it out, I had the enum from PyPI which doesn't have the iterator function built in) here is my solution to the problem

from enum import Enum

class Estimators(Enum):
    rsac = 1
    msac = 2

e = Estimators
attributes = [attr for attr in vars(e) if not attr.startswith('__')]

est_help = 'Valid options are: ' + str(attributes).replace('\'','')

print est_help

I get the members of the class using vars since they are stored in dictionary format and then filter out all the members that start with __ and then since the elements of the list show up as strings with ' I replace those with an empty string.

Some of the code can be reduced if I combine my solution with @SigveKolbeinson's answer like so

est_help = 'Valid options are: [{}]'.format( ', '.join( [str(i) for i in vars(Estimators) if not i.startswith('__')]))

The difference is probably that you are using the enum package off PyPI. You can get the backported stdlib version with the enum34 package, and the versions iterating Estimators directly do work in it.

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