CODEWITHSUNDEEP
csvpython-2.7dictionary
RHK-S8
RHK-S8

Reputation: 329

Using Python's csv.dictreader to search for specific key to then print its value

BACKGROUND:

I am having issues trying to search through some CSV files. I've gone through the python documentation: http://docs.python.org/2/library/csv.html about the csv.DictReader(csvfile, fieldnames=None, restkey=None, restval=None, dialect='excel', *args, **kwds) object of the csv module.

My understanding is that the csv.DictReader assumes the first line/row of the file are the fieldnames, however, my csv dictionary file simply starts with "key","value" and goes on for atleast 500,000 lines.

My program will ask the user for the title (thus the key) they are looking for, and present the value (which is the 2nd column) to the screen using the print function. My problem is how to use the csv.dictreader to search for a specific key, and print its value.

Sample Data: Below is an example of the csv file and its contents...

"Mamer","285713:13"

"Champhol","461034:2"

"Station Palais","972811:0"

So if i want to find "Station Palais" (input), my output will be 972811:0. I am able to manipulate the string and create the overall program, I just need help with the csv.dictreader.I appreciate any assistance.

EDITED PART:

    import csv

    def main():
        with open('anchor_summary2.csv', 'rb') as file_data:
        list_of_stuff = []
        reader = csv.DictReader(file_data, ("title", "value"))
            for i in reader:
              list_of_stuff.append(i)
            print list_of_stuff

    main()

Upvotes: 3

Views: 35337

Answers (2)

John P
John P

Reputation: 21

The currently accepted answer is fine, but there's a slightly more direct way of getting at the data. The dict() constructor in Python can take any iterable.

In addition, your code might have issues on Python 3, because Python 3's csv module expects the file to be opened in text mode, not binary mode. You can make your code compatible with 2 and 3 by using io.open instead of open.

import csv
import io

with io.open('anchor_summary2.csv', 'r', newline='', encoding='utf-8') as f:
    data = dict(csv.reader(f))

print(data['Champhol'])

As a warning, if your csv file has two rows with the same value in the first column, the later value will overwrite the earlier value. (This is also true of the other posted solution.)

If your program really is only supposed to print the result, there's really no reason to build a keyed dictionary.

import csv
import io


# Python 2/3 compat
try:
    input = raw_input
except NameError:
    pass


def main():
    # Case-insensitive & leading/trailing whitespace insensitive
    user_city = input('Enter a city: ').strip().lower()

    with io.open('anchor_summary2.csv', 'r', newline='', encoding='utf-8') as f:
        for city, value in csv.reader(f):
            if user_city == city.lower():
                print(value)
                break
        else:
            print("City not found.")

if __name __ == '__main__':
    main()

The advantage of this technique is that the csv isn't loaded into memory and the data is only iterated over once. I also added a little code the calls lower on both the keys to make the match case-insensitive. Another advantage is if the city the user requests is near the top of the file, it returns almost immediately and stops looking through the file.

With all that said, if searching performance is your primary consideration, you should consider storing the data in a database.

Upvotes: 2

michaelb958--GoFundMonica
michaelb958--GoFundMonica

Reputation: 4708

The documentation you linked to provides half the answer:

class csv.DictReader(csvfile, fieldnames=None, restkey=None, restval=None, dialect='excel', *args, **kwds)

[...] maps the information read into a dict whose keys are given by the optional fieldnames parameter. If the fieldnames parameter is omitted, the values in the first row of the csvfile will be used as the fieldnames.

It would seem that if the fieldnames parameter is passed, the given file will not have its first record interpreted as headers (the parameter will be used instead).

# file_data is the text of the file, not the filename
reader = csv.DictReader(file_data, ("title", "value"))
for i in reader:
  list_of_stuff.append(i)

which will (apparently; I've been having trouble with it) produce the following data structure:

[{"title": "Mamer", "value": "285713:13"},
 {"title": "Champhol", "value": "461034:2"},
 {"title": "Station Palais", "value": "972811:0"}]

which may need to be further massaged into a title-to-value mapping by something like this:

data = {}
for i in list_of_stuff:
  data[i["title"]] = i["value"]

Now just use the keys and values of data to complete your task.

And here it is as a dictionary comprehension:

data = {row["title"]: row["value"] for row in csv.DictReader(file_data, ("title", "value"))}

Upvotes: 2

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