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Python Primer

Q1 Why bool,int,float,str,tuple,frozenset are immutable?​

Initially, I thought bool,int,float are mutable, let's see the code:

a = 1
a += 1
print(a)

The result of a is 2, which means the value of a is changed.But the type of a is still int.

But, "a class is immutable if each object of that class has a fixed value upon instantiation that cannot subsequently be changed." Let's see the code:

a = 10
print(id(a))
a += 1
print(id(a))

the result is:

140734261726800
140734261726832

the id of a is changed, which means the value of a is changed.

Q2 Int()​

use int() will return the value based upon an existing value of another type.For example: int(1.12) =1, int(-1.99) =-1,int(-3.01)=-3, and int()=0 the trucated part is the integer part of the value.

Q3 Tuple class​

when there is only one element in the tuple, like(1,), we must use a comma, otherwise, it will be int type, and becomes (1).

Q4 Yield​

The defference between return and yield: the yield is forward-looking and the return is backward-looking.

So it means that, yield allow to step by step to aquire the results, but the return must gain the results right away.