A python code read each line in the input file, check if there is no negative number and ignore all space

Assuming the input file has one number per line, here is an example code:

with open('input.txt', 'r') as f:
    for line in f:
        # Remove all spaces
        line = line.replace(" ", "")
        # Check if line is a negative number
        if line.startswith("-"):
            print("Error: Negative number found.")
            continue
        # Convert line to integer and do something with it
        num = int(line)
        print("The number is:", num)

This code uses a `with` statement to automatically close the file once we're done reading from it. We then iterate over each line in the file using a `for` loop.

Inside the loop, we first remove all spaces from the line using the `replace` method. We then check if the line is a negative number by using the `startswith` method to see if it begins with a `-` sign. If it does, we print an error message and use the `continue` statement to skip to the next iteration of the loop.

If the line is not a negative number, we convert it to an integer using the `int` function and store it in the `num` variable. We can then do something with this number, such as printing it out.