Python语法--Variable

学习笔记：本文属于自用笔记，仅供参考。

Variable Naming

Nota Bene: Python is a case sensitive programming language. Be aware of this when you first name and subsequently use your variables. For instance, leapyear, Leapyear, leapYear, and LeapYear will actually be viewed as 4 different variables by Python even if you intended the same thing, a “leap year,” but simply mistyped.

Variable Assignment

Variable are lines of code that do something. The most common type of statement you’ll probably come across are assignment statements.

N.B.: Although we can understand simple programs using this description, and many programming languages, including C, C++, and Java, do exactly this, Python doesn’t. Instead, a variable points to a storage location containing its current value, and so does every variable with that same value. If its value is changed by assignment then, instead of changing its storage location to that value, a new location is set to the value if it’s not already in memory, then the variable is changed to point to that new location–but other variables pointing to the old location still point there. Moreover, Python assumes small integers (-5 to +256) are very likely to be used, so they’re automatically stored when the system is initialized, ready to be used (a process called memoization). So, to be quite accurate:

x = 5  # sets variable x to point to the location storing 5 (memoized during system initialization)

y = x  # sets y to point to the location x presently does (so the location storing 5)

x = x + 1 # sets variable x to point to the location storing 6 (memoized during system initialization)--but y still points to 5

y = 3.14  # stores 3.14 in a new memory location unless it has already been used and so is already available (has been memoized), and sets y to point to it 

w = y  # sets w to point to the same value y presently points to (3.14)

z = 3.14  # sets z to point to 3.14, the same value as y and w presently point to

So instead of thinking of a variable being a named location whose value gets changed, it’s more accurate to think of a variable as a name on a sticky note that gets moved and stuck to a location storing its new value (after storing that value if it hasn’t been used previously). Note that we have just one location storing value 3.14, but three variables (w, y, and z) presently share that value. If any one of them changes, the value in that location doesn’t change; instead, that variable now points to a different location containing that new value and the other two still “point to” 3.14.

Assigning from Mathematical Expressions

More complex mathematical expressions can be evaluated for variable assignment than just numeric literals. This can be achieved by way of the following operators:

• Addition ( + )
  • x = 5 + 10
• Subtraction ( - )
  • x = 5 - 10
• Multiplication ( * )
  • x = 5 * 10
• Power ( ** )
  • Exponentiation ( 510 )x = 5**10
  • The operator can also be utilized for root calculations since roots are just fraction exponents ( 10 = 101/2 )`x = 10.5`
• Division ( / )
  • x = 5 / 10
• Modulus ( % )
  • Used to return the remainder of a division instead of the quotient answer ( 10 % 3 = 1, since 3 * 3 = 9 and 10 - 9 leaves a remainder 1 )
  • x = 10 % 3

Nota Bene: There is a marked difference between integer number division and floating point number division. Because Python treats these two numeric types distinctly different, you might get erroneous answers when doing division operations without precaution. (read more on Data Types: Other Caveats below)

Assigning from User Input–raw_input()

The command to accept user input is raw_input(). This will make a pop-up box appear on your screen.

Example:

name = raw_input("What is your name, human? ")
print "Hello", name + "! I am Python :D"

Note Chandrama:In Python 3.x,input() replaces raw_input(), for input from the console.Also remember to use the print() function with Python 3.4, including parentheses, for output to the console, instead of the old print command that did not require parentheses.So the example above will be in Python 3x as follows:

name = input("What is your name, human? ")
print ("Hello", name + "! I am Python :D")

Data Types—type()

The data types we have learned about so far are:

• Numeric types
  • Integer numbers
    A whole number, i.e. a number that is not a fraction
    Ex: 1, 5, 33, -24, 5000
  • Floating-point numbers
    A number with a fraction or decimal point
    17.34, 98.2, -768.001, 0.2
• Strings
  • A collection of characters
  • This can be alphabetic characters, special characters, or even numeric characters (but saved as type string, enclosed in “quotes”)
  • Ex: “Alphabetic”, “ch@r@cter$!!!”, “1234567890”

    Nota Bene: If you are unsure of what data type a particular variable, constant, or literal is, you can ask Python by using thetype() function.

String concatenation example:

print "Hello" + "Coursera!"

#Output: HelloCoursera!

Note that when using concatenation with the + operator, sentence spacing isn’t automatically allotted for. Whereas print “Hello”, “Coursera!” would have output “Hello Coursera!”. Also the multiplication * operator can be used for multiple string concatenation:

print "Free Education!", "Hip-hip-hooray! " * 3 + "Thank you Coursera!"

#Outputs: Free Education! Hip-hip-hooray! Hip-hip-hooray! Hip-hip-hooray! Thank you Coursera!

Type Conversion–int()

Type-casting is a universal programming notion of converting the value of a variable from one type into another. In Python, this is achieved by utilizing functions such asint(), float(), or str().

x = raw_input("Enter a number: ")
y = raw_input("Enter another number: ")
z = x + y
print "The sum of", x, "and", y, "is",  z

#Output:
# Enter a number: 23
# Enter another number: 45
# The sum of 23 and 45 is 2345

z = int(x) + int(y)
print "The correct sum actually was", z

#Output: The correct sum actually was 68

Debugging

Here are a few pointers on debugging.

1. Avoid spaces in between variable names. Python thinks there are two operands. Example:

   >>>Madam Medousa = "lets start the bout"
   SyntaxError: invalid syntax
   
   #correct syntax
   >>>madam_medousa = "lets start the bout"
   >>>print madam_medousa
   "lets start the bout"

   Common Syntax Errors. 1) SyntaxError: invalid syntax 2) SyntaxError: invalid token

2. Runtime Error: using a variable before it is defined. Example:

    >>>cat= "Garfield"
    >>>food = "burgers"
    >>>Garfield + food
   NameError: name"Garfield" is not defined
   
   # "Garfield" is a string in the variable cat.
   # Hence, Garfield is not a defined as a mnemonic variable.

3. Case sEn5iTiVity (sensitivity!) matters:

   >>>Olive = 5
   >>>olive = "olive oil"
   >>>oLiVe = 7
   
    # variable names are case sensitive,
   # all of these variations of olive are separate variables.

参考链接

Pythonlearn:resources-week01