# coding: utf-8 # # Python Introduction: Names and Values # Define and reference a variable: # In[22]: a = 3*2 + 5 # In[23]: a # In[24]: a = "interesting"*3 # In[25]: a # No type declaration needed! # # (But values still have types--let's check.) # In[26]: type(a) # Python variables are like *pointers*. # # (if that word makes sense) # In[27]: a = [1,2,3] # In[28]: b = a # In[29]: b.append(4) # In[30]: b # In[31]: a # You can see this pointer with `id()`. # In[33]: print(id(a), id(b)) # The `is` operator tests for object sameness. # In[34]: a is b # This is a **stronger** condition than being equal! # In[36]: a = [1,2,3] b = [1,2,3] print("IS ", a is b) print("EQUAL", a == b) # What do you think the following prints? # In[38]: a = [1,2,3] b = a a = a + [4] print(b) # In[39]: a is b # Why is that? # ----- # How could this lead to bugs? # ---------- # * To help manage this risk, Python provides **immutable** types. # # * Immutable types cannot be changed in-place, only by creating a new object. # # * A `tuple` is an immutable `list`. # In[40]: a = [1,2,3] type(a) # In[41]: a = (1,2,3) type(a) # Let's try to change that tuple. # In[42]: a[2] = 0 # *Bonus question:* How do you spell a single-element tuple? # In[43]: a = (3,) type(a)