Python Basics: Datatypes and Key Functions

An introductory guide covering Python’s basic data types and essential functions.

1. Data Types

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a. Dictionary (dict)

• Definition:
  d = {key1: value1, key2: value2, ...}
  Defined using curly braces {}. Key-value pairs are separated by colons :, and multiple elements are separated by commas ,.
• Constraints:
  • Keys must be unique and are case-sensitive. Assigning a value to an existing key will overwrite the old value.
  • Dictionary elements are unordered.
  • Values can be of any data type.
• Operations:
  • Assignment: d['newkey'] = newvalue
  • Delete an element: del d['newkey']
  • Clear dictionary: d.clear()

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b. List (list)

• Definition (think of it as a dynamic array):
  l = [a, b, c, ...]
  Defined using square brackets [], with elements separated by commas ,.
• Constraints:
  • Elements are ordered.
  • For a non-empty list, the first element is l[0]. In a list with n elements, the last element is l[n-1] (or l[-1]).
  • Lists allow negative indexing. Negative values count backward from the end of the list.
  • Values can be of any data type.
• Slicing:
  Specify two index values to get a subset: l[0:3] returns the first 3 elements of the list. The start and end indexes can be omitted, defaulting to 0 and -1 respectively.
• Operations:
  • Add:
    • l.append('new'): Appends an element to the end of the list.
    • l.insert(1, 'new'): Inserts an element at a specified index. Elements after it will shift one position to the right.
    • l.extend(["two", "elements"]): Concatenates two lists. Note that the argument for extend must be a list, and new elements are added to the end.
  • Search:
    • l.index('a'): Finds the first occurrence of a value in the list and returns its index. If the value is not found, Python raises an exception.
    • 'a' in l: Tests if a value is in the list. Returns True if it exists, otherwise False.
  • Delete:
    • l.remove('a'): Removes the first occurrence of the value.
    • l.pop(): Removes and returns the last element of the list.
  • List Operators:
    • +: Concatenates lists (similar to extend, though extend is faster for large lists).
    • *: Repeats the list a specified number of times.

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c. Tuple (tuple)

A tuple is simply an immutable list.

• Definition:
  t = ('a', 'b', 'c')
  Defined using parentheses (), with elements separated by commas ,.
• Constraints:
  • You cannot add, remove, or modify elements in a tuple.
• Advantages:
  • Tuples are faster than lists; perfect for constant values.
  • Safer. You can easily convert a tuple to a list (but not vice versa, as lists are mutable).
  • Can be used as dictionary keys (lists cannot, because they are mutable).
  • Tuples can be used for string formatting.

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2. Function Definition

def function(parameter1, parameter2, ...):
    # Function body requires indentation

Essential Built-in Functions

• type 函数: Returns the data type of any object.
• str 函数: Casts data into a string.
• dir 函数: Returns a list of valid attributes and methods for an object.
• callable 函数: Checks if an object is callable (e.g., a function or method).
• getattr 函数: Retrieves a reference to an attribute/method by name; a core feature for reflection/introspection and dispatching.

Other Tips

• Python’s built-in functions reside in the __builtin__ module.
• Recommended learning resource: Dive into Python—an excellent book!