How to Append or Write Only Once to a File in Python

Question

I have to open a file and loop through the list. Then I have to print the results and append/write certain lines to the same file. I want to be able to run the code multiple times, but I do not want to append certain lines multiple times, but only once. The question is – how to append/write only once? Here is the code:

#include <algorithm>
#include <ctime>
#include <iostream>

int main()
{
  // Generate data
  const unsigned arraySize = 32768;
  int data[arraySize];

  for (unsigned c = 0; c < arraySize; ++c)
    data[c] = std::rand() % 256;

  // !!! With this, the next loop runs faster.
  std::sort(data, data + arraySize);

  // Test
  clock_t start = clock();
  long long sum = 0;
  for (unsigned i = 0; i < 100000; ++i)
  {
    for (unsigned c = 0; c < arraySize; ++c)
    {  // Primary loop.
      if (data[c] >= 128)
        sum += data[c];
    }
  }

  double elapsedTime = static_cast<double>(clock()-start) / CLOCKS_PER_SEC;

  std::cout << elapsedTime << '\n';
  std::cout << "sum = " << sum << '\n';
}

I tried to use the break method, but it doesn’t help.

Answer

To append/write only once to the file, you can use a boolean flag variable to track whether a certain line has been written to the file or not.

Here’s an example of how you can modify your code to achieve this:

kitty = 500

requests = []

file = open("loan_requests.txt", "r+")

requests = file.readlines()

# Initialize a flag variable to track whether the "Outstanding Request" line has been written
request_unpaid = False

for item in requests:
  if int(item) < kitty and kitty > 0:
    kitty = kitty - int(item)
    loan = int(item)
    file.write("Request of {} paid in full.".format(loan))
    print(loan, "- Paid!")

  elif int(item) > kitty and kitty > 0:
    kitty = kitty - int(item)
    loan = int(item) + kitty
    file.write("Request of {} could not be paid in full.Partial payment of {} made.".format(item, loan))
    print(int(item), "request cannot be processed in full (Insufficient funds available). Amount paid:", loan)
    
  elif int(item) > kitty and kitty <= 0 and not request_unpaid:
    # Write the "Outstanding Request" line only if it has not been written before
    file.write("Outstanding Request:{}".format(item))
    request_unpaid = True
    print("Request of", int(item), "is UNPAID!")

file.close()

With this modification, the “Outstanding Request” line will be written to the file only once, even if multiple requests are unpaid.