assert

Learning objectives

• Understand design by contract
• Understand algorithm names increase expressiveness of code
• Understand difference between if and assert in function writing

The role of assert within functions

Within functions, assert is used for:

• as a stub
• to do things in debug mode only
• to document assumptions a developer makes

assert differs between debug and release

$ cat assert.py 
assert 1 == 2

$ python -O assert.py 

$ python assert.py 
Traceback (most recent call last):
  File "/home/richel/assert.py", line 1, in <module>
    assert 1 == 2
AssertionError

assert as a stub

def align(dna_sequences):
    """Align the DNA sequences"""
    assert len(dna_sequences) == 2 # TODO

assert in debug mode

def align_two_dna_sequences(dna_sequences):
    """Internal function to align two DNA sequences"""
    assert len(dna_sequences) == 2 # TODO

Superior to documentation, as it cannot be ignored.

'Assert liberally to document internal assumptions and invariants' @sutter2004cpp chapter 68.

assert to document assumptions a developer makes

def align_two_dna_sequences(dna_sequences):
    """Align the DNA sequences"""
    # ....
    results = ["AAAA", "AAC-"] # Should be result of calculation
    assert len(results[1]) == len(results[2])