Practical Programming, 2nd Edition

Nesting Loops in Loops

The block of statements inside a loop can contain another loop. In this code, the inner loop is executed once for each item of list outer:

>>> outer = ['Li', 'Na', 'K']
>>> inner = ['F', 'Cl', 'Br']
>>> for metal in outer:
...     for halogen in inner:
...         print(metal + halogen)
...
...
LiF
LiCl
LiBr
NaF
NaCl
NaBr
KF
KCl
KBr

The number of times that function print is called is len(outer) * len(inner). In Table 12, ​Nested Loops Over Inner and Outer Lists ​, we show that for each iteration of the outer loop (that is, for each item in outer), the inner loop executes three times (once per item in ...