从文件指针读取文件而不使用 readlines

Question

can someone help me fix this code I keep getting this error, I'm struggling.

for every code I practice, I keep getting the same error

This is the customers.txt file for this question that we are supposed to work with

12345,Tom,Black,300.00,1998-01-30
23456,Alice,Smith,1200.50,1998-02-20
14567,Jane,White,900.00,1998-07-01
43564,Weilin,Zhao,450.25,1998-01-03
45432,Bina,Mehta,278.95,1998-03-21

def customer_first(fh):
    record = None
    lines = fh.readlines()
    i = 0
    while i < len(lines):
        line = lines[i]
        values = line.rstrip('\n').split(',')
        if record == None or values[4] < record[4]:
            record = values
        i += 1
    return record

...

Sample testing:

Find customer with earliest sign-in:

['43564', 'Weilin', 'Zhao', '450.25', '1998-01-03']

this is the error I keep getting down below.

Test for calls to forbidden Python functions: 'readlines'

ERROR:

function should not call 'readlines'

these are all the things I'm being tested on in my code:

Test various parameters: 'customers file variable'

OK

Test for calls to forbidden Python functions: 'input'

OK

Test for calls to forbidden Python functions: 'print'

OK

Test for calls to forbidden Python functions: 'readlines'

ERROR:

function should not call 'readlines'

Test for calls to forbidden Python functions: 'open'

OK

Test for calls to forbidden Python functions: 'close'

OK

Test function docstring documentation:

OK

Test for multiple returns:

OK

Test that the function does not use 'for':

OK

Test that the function does not hard code the length of the file:

OK

Test for misuse of function name:

OK

Answer 1

Your grading system might expect this, as an alternative to using .readlines ():

def customer_first(fh):
    record = None
    lines = fh.read().split('\n')
    i = 0
    while i < len(lines):
        line = lines[i]
        values = line.split(',')
        if record == None or values[4] < record[4]:
            record = values
        i += 1
    return record

Here, we call .read() to read the whole file in at once, and then .split() it into individual lines for each line break. This is essentially what .readlines() probably does under the hood, and is the most direct replacement for your code that doesn't use that method.

---

Your grading system might also be expecting you to do this:

def customer_first(fh):
    record = None
    i = 0
    line = fh.readline()
    while line != '':
        values = line.rstrip('\n').split(',')
        if record == None or values[4] < record[4]:
            record = values
        line = fh.readline()
    return record

File.readline() is similar to File.readlines() except that it returns only one line, and returns an empty string if you've reached the end of the file.

---

Another alternate solution is to iterate through the file with next() , and use a try / except block to catch a StopIteration (which is how for loops actually work under the hood). This is a more complicated, more advanced, and more niche method, though, which I think it's unlikely your teacher expects you to be using here unless this is something you've just learned about:

def customer_first(fh):
    record = None
    i = 0
    try:
        line = next(fh)
        while line != '':
            values = line.rstrip('\n').split(',')
            if record == None or values[4] < record[4]:
                record = values
            line = next(fh)
    except StopIteration
        return record

---

I'd like to impress upon you that using for loops when possible is generally good practice, using .readlines() in general is usually not bad practice when simply iterating over the file in a for loop isn't sufficient, and you shouldn't take this assignment as a sign to avoid doing these things in the future. If the assignment doesn't tell you what tool you're supposed to be using, and the recent lectures you've had in class haven't covered one of the above intending to clue you into it, then it's my opinion this is a bad assignment.

Also, your code probably has a logic error (try record == None and values[4] < record[4] ), but that's not the issue you're asking about, and once the grading system starts actually running your code, you'll get different error messages that would probably lead you to figuring it out anyway.

Answer 2

If you really can't use readlines() or the csv module then certainly you can loop over the file such as:

data=[line.split(',') for line in fh.read().splitlines()]
>>> data
[['12345', 'Tom', 'Black', '300.00', '1998-01-30'], ['23456', 'Alice', 'Smith', '1200.50', '1998-02-20'], ['14567', 'Jane', 'White', '900.00', '1998-07-01'], ['43564', 'Weilin', 'Zhao', '450.25', '1998-01-03'], ['45432', 'Bina', 'Mehta', '278.95', '1998-03-21']]

Since those are ISO 8601 style dates, no conversion from text needed. You can get the min this way:

>>> min(data, key=lambda sl: sl[4])
['43564', 'Weilin', 'Zhao', '450.25', '1998-01-03']