In [3]: 
a = [1, 123, 456]
In [4]: 
print a

[1, 123, 456]
In [8]: 
a_squares = []
for v in a:
    a_squares = a_squares + [v*v]
print a_squares

[1, 15129, 207936]
In [9]: 
a_squares = []
for v in a:
    a_squares.append(v*v)
print a_squares

[1, 15129, 207936]
In [10]: 
a_squares = [ v*v for v in a ]
print a_squares

[1, 15129, 207936]
In [11]: 
d = { 'bob': 20, 'joe': 21 }
In [12]: 
print d

{'bob': 20, 'joe': 21}
In [13]: 
d['bob']
Out[13]:
20
In [14]: 
d['bob'] = 21
In [15]: 
print d

{'bob': 21, 'joe': 21}
In [16]: 
d['jane'] = 20
In [17]: 
print d

{'jane': 20, 'bob': 21, 'joe': 21}
In [19]: 
print a

[1, 123, 456]
In [22]: 
for p,age in d.iteritems():
    print "Hello "+p+", you are "+str(age)

Hello jane, you are 20
Hello bob, you are 21
Hello joe, you are 21
In [25]: 
message = { p: "Hello, you are "+str(age)   for p,age in d.iteritems() }
In [27]: 
print message

{'jane': 'Hello, you are 20', 'bob': 'Hello, you are 21', 'joe': 'Hello, you are 21'}
In [28]: 
message = { p: "Hello, you are "+str(age)   for p,age in d.iteritems() if age > 20 }
In [29]: 
print message

{'bob': 'Hello, you are 21', 'joe': 'Hello, you are 21'}
In [30]: 
a
Out[30]:
[1, 123, 456]
In [31]: 
a[1] = 1234567
In [32]: 
print a

[1, 1234567, 456]
In []: