Python Control Flow Cheatsheet

Python Control Flow Cheatsheet#

For loops#

Use for loops to iterate over items in a list. The same code will run multiple times, with the loop variable changing each time the loop runs.

scores = [4, 6, 3]  # we will iterate over this list
for score in scores:  # this loop runs three times, with score changing
    print(score)  # on each loop, print what the score variable is currently

4
6
3

Use code within the for loop (indented under the for ... in ...: statement) to do something with the loop variable. For example, say we want to calculate a sum over all the items in a list.

total_score = 0  # to calculate a total, start at zero
for score in scores:  # loop over all scores
    total_score += score  # add this score to the total
print(total_score)

13

Looping over indices#

Use range and len to loop over indices of a list.

print(range(3))  # use range to get a range of numbers
print(list(range(3)))  # don't see the all numbers unless converted to a list first

range(0, 3)
[0, 1, 2]

for i in range(3):
    print(i)  # in a for loop, range will generate each number in the range

0
1
2

for i in range(len(scores)):  # use len to get the number of items in a list
    print(scores[i])  # each value of i is an index in scores

4
6
3

We can calculate the same total score as before, but now using indices to index into the scores list.

total_score = 0
for i in range(len(scores)):
    total_score += scores[i]
print(total_score)

13

Looping using enumerate and zip#

Use the enumerate function to get both indices of a list and their corresponding items.

total_included = 0
for i, score in enumerate(scores):
    if i >= 2:  # exclude the first two trials
        total_included += score  # add to total if included
print(total_included)

3

Use the zip function to iterate over two or more lists simultaneously.

total_combined = 0
scores_test1 = [4, 3, 7]  # scores for each participant from test 1
scores_test2 = [8, 2, 5]  # corresponding scores for test 2
for score1, score2 in zip(scores_test1, scores_test2):
    total_combined += score1 + score2  # add the two scores for this participant
mean_combined = total_combined / len(scores_test1)
print(mean_combined)

9.666666666666666

Using conditionals with loops#

Use a combination of for loops and if statements to perform more complicated calculations, such as filtering out invalid data.

response_times = [0.83, 0.92, 1.24, 0.97, 0.65, 0.22]  # response times in seconds
total_included = 0  # total of included times so far
n_included = 0      # number of included trials so far
for rt in response_times:
    if rt >= 0.3:  # exclude trials with response times less than 0.3 s
        total_included += rt
        n_included += 1
mean_included = total_included / n_included

Comprehensions#

Use list comprehensions to quickly create a list or make a modified copy of an existing list.

For example, say we want to generate standard subject identifiers of the form sub-XXX. First, let’s see how we can use a normal for loop to do this. The :03d is a format specifier that indicates we want to take the number (e.g., 1) and pad it with zeros to make it three digits (e.g., 001).

subject_ids = []
for number in range(1, 6):
    subject_ids.append(f"sub-{number:03d}")
print(subject_ids)

['sub-001', 'sub-002', 'sub-003', 'sub-004', 'sub-005']

Using a list comprehension, we can write the same thing more compactly.

subject_ids = [f"sub-{number:03d}" for number in range(1, 6)]
print(subject_ids)

['sub-001', 'sub-002', 'sub-003', 'sub-004', 'sub-005']

To exclude some items from your list, use an if statement with the not and in operators (in checks if an item is somewhere in a list).

excluded = [2, 5]  # subject numbers 2 and 5 should be excluded
subject_ids = [f"sub-{number:03d}" for number in range(1, 6) if number not in excluded]
print(subject_ids)

['sub-001', 'sub-003', 'sub-004']

List comprehensions can also be used to make a modified copy of an existing list. Here, we take the list of subject identifiers we just made and get just the numeric part, without the "sub-" part. First, let’s see how this works if we use a standard for loop.

subject_codes = []
for id in subject_ids:
    subject_codes.append(id.split("-")[1])
print(subject_codes)

['001', '003', '004']

We can write the same thing using a list comprehension instead.

subject_codes = [id.split("-")[1] for id in subject_ids]
print(subject_codes)

['001', '003', '004']

Exceptions#

Exceptions are used to indicate when something unexpected has happened that makes it difficult or impossible to run a program normally.

When writing a function, use the raise keyword with an error function to indicate when your program identifies that something is wrong.

def generate_id(number):
    if number < 1:
        raise ValueError("Number must be greater than zero.")
    id = f"sub-{number:03d}"
    return id

Use the error function that best fits the case you are testing for. Common error types are ValueError (an input to a function has an inappropriate value), TypeError (an input has an inappropriate data type), and RuntimeError (other unexpected problems).

print(generate_id(2))
print(generate_id(100))
# generate_id(0)  # will raise an exception and halt code execution

sub-002
sub-100

When calling a function, use a try/except block to avoid crashing your program, and do something else instead.

number = 0
try:
    id = generate_id(number)  # try to run this...
except:
    id = None  # run this if there is an exception
print(id)

None
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