Q & A

2021.2.23 Newline Characters in Text Files

Q: Windows/DOS uses Carriage Return and Line Feed ("\r\n") as a line ending, while Unix uses just Line Feed ("\n"). Why this example in Jupyter Notebook shows that the string returned by infile.read() only contains "\n". Is there something wrong?

1. Use Notepad to create a text file "test.txt"

   A
   B
   C

   Note that you should press ENTER after "C" so that the last line contains a line ending.
2. "DIR test.txt" shows that the file size is 9 bytes.
3. "od -t d1 name.txt" shows the ASCII code of each byte.
   "od -c name.txt" shows the nine characters.
4. Now try to read the text file into Python:

   infile = open("test.txt", "r")
   s = infile.read()
   print(len(s), repr(s))
   infile.close()

5. You see the string length is 6, and its content is "A\nB\nC\n".
6. This must imply that, Python automatically perform a conversion when it is handling text files.
7. Let's do another experiment. This time, we open the file as a binary file:

   infile = open("test.txt", "br")
   b = infile.read()
   print(len(b), repr(b))
   infile.close()

8. This time the contents of b contain "\r\n", and the total size is 9 bytes.

2021.2.23 self

Q: What's the meaning of the "self" parameter in a class method? Is it used to specify this is a private member function?

1. The "self" in Python is quite different from "private" in C++. The "private" in C++ specifies that it is a private member, so it can only be accessed by member functions or friends of that class.

   #include <iostream>
   using std::cin;
   using std::cout;
   using std::endl;
   
   class Student {
   public:
       Student(int y) { year = y; }
       int getYear() { return year; }
   private:
       int year;
   };
   
   class Teacher {
   public:
       int calcAge(Student a) {
           int age;
           // age = 2021 - a.year;
           age = 2021 - a.getYear();
           return age;
       }
   };
   
   int main()
   {
       Student alice(1999);
       Student bob(2000);
       Teacher ted;
   
       cout << ted.calcAge(alice) << endl;
       cout << ted.calcAge(bob) << endl;
       return 0;
   }

2. If we translate this C++ code to Python, you can see that both year and getYear() can be accessed.

   class Student:
       def __init__(self, y):
           self.year = y
       def getYear(self):
           return self.year
   
   class Teacher:
       def calcAge(self, a):
           age = 2021 - a.year
           # age = 2021 - a.getYear()
           return age
   
   def main():
       alice = Student(1999)
       bob = Student(2000)
       ted = Teacher()
   
       print( ted.calcAge(alice) )
       print( ted.calcAge(bob) )
   
   main()

3. P.790 Python's self is the same as the this pointer in C++ or Java, but self is always explicit in both headers and bodies of Python methods to make attribute accesses more obvious.
4. alice.getYear() is automatically translated to Student.getYear(alice)

   class Student:
       def __init__(self, y):
           self.year = y
       def getYear(self):
           return self.year
   
   class Teacher:
       def __init__(self, n):
           self.age = n
       def calcAgeDiff(self, a):
           studentAge = 2021 - a.year
           return self.age - studentAge
   
   def main():
       alice = Student(1999)
       ted = Teacher(28)
   
       print( alice.getYear(), Student.getYear(alice) )
       print( ted.calcAgeDiff(alice), Teacher.calcAgeDiff(ted, alice) )
   
   main()

5. If you really want to define a private member in a Python class, prefix the member name with double underscore "__".

   class Student:
       def __init__(self, y):
           self.__year = y
       def getYear(self):
           return self.__year
   
   def main():
       alice = Student(1999)
   
       print( alice.getYear() )
       print( alice.__year )
   
   main()