CODEWITHSUNDEEP
python
Usman
Usman

Reputation: 21

Compare a list and a file for match in python

I want to loop over each element in a List and check if that element matches any of the products in the text file. I have done this:

print("Opening a file")
pro_file = open("inventory.txt", "r")   #open text file for reading

#print(pro_file.read())    #read text file line by line
productList = ['chips', 'biscuits', 'pasta', 'cheese', 'bread', 'rice', 'honey', 'butter', 'cake', 'salt'];

for key in productList:
    for line in pro_file.readlines():
        if key in line:
            print("found" + key)

The nested for loops are only giving the match of first item in the productList. I have stated learning python few days back, so any help would be appreciated.

Upvotes: 0

Views: 1222

Answers (2)

Troy Hoffman
Troy Hoffman

Reputation: 193

The problem is that once you call readlines(), the end of file is reached and the next time you call it on the same open file, it won't return anything. A quick fix would be to just swap the two for statements like this:

for line in pro_file.readlines():
    for key in productList:

However, this will have problems with a large file since readlines() creates a list of all lines in the file and stores that in memory. So, you could try this. I've added comments to explain some of the other changes.

# Per PEP8, variable names should be all lower case with underscores between words
product_list = ['chips', 'biscuits', 'pasta', 'cheese', 'bread', 'rice', 'honey', 'butter', 'cake', 'salt'];

# This is a context manager. It will ensure the file is closed when the code block is finished
# No need to include 'r' when opening a text file as read-only. It's the default.
with open("inventory.txt") as pro_file:
    for this_line in pro_file:
        # A text file can be iterated through one line at a time this way, without loading everything into memory
        for product in product_list:
            if product in this_line:
                # Using format is cleaner and easier to read than + and will work even if the value is a number
                print('Found: {}'.format(product))
                # Since you found a product, you can stop checking products. Break will stop the for product loop.
                break
        else:  # no break
            # What the heck is this? An else in a for loop?
            # In Python, this means "do this if you didn't hit a break statement," and it's very useful.
            # I usually add the comment "no break" after it so it's a little more clear that it was intentional.
            print('Did not find any products in this line')

Upvotes: 1

Francesco Montesano
Francesco Montesano

Reputation: 8658

The first time you call pro_file.readlines(), you reach the end of the file. The second time you call it, there is nothing more to read.

Just read the file once:

with open("inventory.txt", "r") as f:
    pro_file = f.readlines()

then loop the pro_file list

for key in productList:
    for line in pro_file:
        if key in line:
            print("found" + key)

However if you just want to know if one of 

productList = ['chips', 'biscuits', 'pasta', 'cheese', 'bread', 'rice', 'honey', 'butter', 'cake', 'salt'];

is in pro_file and you don't care where it is, you can simplify the above code in the following way:

for key in productList:
    if key in pro_file:
        print("found" + key)

Upvotes: 2

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