Python: Writing to file using while loop fails with no errors given

Question

I am attempting to collect only certain type of data from one file. After that the data is to be saved to another file. The function for writing for some reason is not saving to the file. The code is below:

def reading(data): file = open("model.txt", 'r') while (True): line = file.readline().rstrip("\n") if (len(line) == 0): break elif (line.isdigit()): print("Number '" + line + "' is present. Adding") file.close() return None def writing(data): file = open("results.txt", 'w') while(True): line = somelines if line == "0": file.close() break else: file.write(line + '\n') return None file = "model.txt" data = file somelines = reading(data) writing(data)

I trying several things, the one above produced a TypeError (unsupported operand). Changing to str(somelines) did solve the error, but still nothing was written. I am rather confused about this. Is it the wrong definition of the "line" in the writing function? Or something else?

reading returns None, which you store in somelines but don't use anyway. Other than that it doesn't actually do anything. How are you expecting to get the data out of the reading function? — Kemp
– Kemp, Commented Jun 18, 2021 at 13:19

Red · Accepted Answer · 2021-06-18 13:20:27Z

See this line in your writing function:

 file.write(line + '\n')

where you have

 line = somelines

and outside the function you have

somelines = reading(data)

You made your reading function return None. You cannot concat None with any string, hence the error.

ChrisOram · Accepted Answer · 2021-06-18 13:33:25Z

Assuming you want one reading function which scans the input file for digits, and one writing file which writes these digits to a file until the digit read is 0, this may help:

def reading(file_name): with open(file_name, 'r') as file: while True: line = file.readline().rstrip("\n") if len(line) == 0: break elif line.isdigit(): print("Number '" + line + "' is present. Adding") yield line def writing(results_file, input_file): file = open(results_file, 'w') digits = reading(input_file) for digit in digits: if digit == "0": file.close() return else: file.write(digit + '\n') file.close() writing("results.txt", "model.txt")

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