returning None for EOF, updated comments

cs50/cs50.py

@@ -21,7 +21,11 @@ def write(self, x):
 sys.stdout = flushfile(sys.stdout)
 
 def get_char(prompt=None):
-    """Read a line of text from standard input and return the equivalent char."""
+    """
+    Read a line of text from standard input and return the equivalent char;
+    if text is not a single char, user is prompted to retry. If line can't
+    be read, return None.
+    """
     while True:
         s = get_string(prompt)
         if s is None:
@@ -34,7 +38,11 @@ def get_char(prompt=None):
             print("Retry: ", end="")
 
 def get_float(prompt=None):
-    """Read a line of text from standard input and return the equivalent float."""
+    """
+    Read a line of text from standard input and return the equivalent float
+    as precisely as possible; if text does not represent a double, user is
+    prompted to retry. If line can't be read, return None.
+    """
     while True:
         s = get_string(prompt)
         if s is None:
@@ -50,7 +58,11 @@ def get_float(prompt=None):
             print("Retry: ", end="")
 
 def get_int(prompt=None):
-    """Read a line of text from standard input and return the equivalent int."""
+    """
+    Read a line of text from standard input and return the equivalent int;
+    if text does not represent an int, user is prompted to retry. If line
+    can't be read, return None.
+    """
     while True:
         s = get_string(prompt);
         if s is None:
@@ -69,7 +81,11 @@ def get_int(prompt=None):
 
 if sys.version_info.major != 3:
     def get_long(prompt=None):
-        """Read a line of text from standard input and return the equivalent long."""
+        """
+        Read a line of text from standard input and return the equivalent long;
+        if text does not represent a long, user is prompted to retry. If line
+        can't be read, return None.
+        """
         while True:
             s = get_string(prompt)
             if s is None:
@@ -85,11 +101,18 @@ def get_long(prompt=None):
                 print("Retry: ", end="")
 
 def get_string(prompt=None):
-    """Read a line of text from standard input and return it as a string."""
+    """
+    Read a line of text from standard input and return it as a string,
+    sans trailing line ending. Supports CR (\r), LF (\n), and CRLF (\r\n)
+    as line endings. If user inputs only a line ending, returns "", not None.
+    Returns None upon error or no input whatsoever (i.e., just EOF).
+    """
     try:
         if prompt is not None:
             print(prompt, end="")
         s = sys.stdin.readline()
+        if not s:
+            return None
         return re.sub(r"(?:\r|\r\n|\n)$", "", s)
     except ValueError:
         return None