Python вопросы и ответы для интервью

Вопрос 36. How can I find the methods or attributes of an object?

For an instance x of a user-defined class, dir(x) returns an alphabetized list of the names containing the instance attributes and methods and attributes defined by its class.

Вопрос 37. How do I convert a string to a number?

For integers, use the built-in int() type constructor, e.g. int('144') == 144. Similarly, float() converts to floating-point, e.g. float('144') == 144.0.

By default, these interpret the number as decimal, so that int('0144') == 144 and int('0x144') raises ValueError.

int(string, base) takes the base to convert from as a second optional argument, so int('0x144', 16) == 324. If the base is specified as 0, the number is interpreted using Python's rules: a leading '0' indicates octal, and '0x' indicates a hex number. 

Do not use the built-in function eval() if all you need is to convert strings to numbers. eval() will be significantly slower and it presents a security risk: someone could pass you a Python expression that might have unwanted side effects. For example, someone could pass __import__('os').system("rm -rf $HOME") which would erase your home directory. 

eval() also has the effect of interpreting numbers as Python expressions, so that e.g. eval('09') gives a syntax error because Python regards numbers starting with '0' as octal (base 8).

Вопрос 38. How can my code discover the name of an object?

Generally speaking, it can't, because objects don't really have names. Essentially, assignment always binds a name to a value; The same is true of def and class statements, but in that case the value is a callable. Consider the following code:

class A:
  pass
  B = A
  a = B()
  b = a
  print b
  <__main__.A instance at 016D07CC>
  print a
  <__main__.A instance at 016D07CC>

Arguably the class has a name: even though it is bound to two names and invoked through the name B the created instance is still reported as an instance of class A. However, it is impossible to say whether the instance's name is a or b, since both names are bound to the same value.

Generally speaking it should not be necessary for your code to "know the names" of particular values. Unless you are deliberately writing introspective programs, this is usually an indication that a change of approach might be beneficial.

In comp.lang.python, Fredrik Lundh once gave an excellent analogy in answer to this question:
The same way as you get the name of that cat you found on your porch: the cat (object) itself cannot tell you its name, and it doesn't really care -- so the only way to find out what it's called is to ask all your neighbours (namespaces) if it's their cat (object)
....and don't be surprised if you'll find that it's known by many names, or no name at all!

Вопрос 39. Is there an equivalent of C's "?:" ternary operator?

No.

Вопрос 40. How do I convert a number to a string in Python?

• To convert, e.g., the number 144 to the string '144', use the built-in function str().
• If you want a hexadecimal or octal representation, use the built-in functions hex() or oct().
• For fancy formatting, use the % operator on strings, e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'.