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Pandas: From Messy To Lovely. That is methods to make your pandas code… | by Anna Zawadzka | Mar, 2024

Scripting round a pandas DataFrame can flip into a clumsy pile of (not-so-)good outdated spaghetti code. Me and my colleagues use this package deal loads and whereas we attempt to keep on with good programming practices, like splitting code in modules and unit testing, generally we nonetheless get in the best way of each other by producing complicated code.

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I’ve gathered some suggestions and pitfalls to keep away from to be able to make pandas code clear and infallible. Hopefully you’ll discover them helpful too. We’ll get some assist from Robert C. Martin’s basic “Clear code” particularly for the context of the pandas package deal. TL;DR on the finish.

Let’s start by observing some defective patterns impressed by actual life. Afterward, we’ll attempt to rephrase that code to be able to favor readability and management.


Pandas DataFrames are value-mutable [2, 3] objects. Everytime you alter a mutable object, it impacts the very same occasion that you simply initially created and its bodily location in reminiscence stays unchanged. In distinction, whenever you modify an immutable object (eg. a string), Python goes to create a complete new object at a brand new reminiscence location and swaps the reference for the brand new one.

That is the essential level: in Python, objects get handed to the operate by project [4, 5]. See the graph: the worth of df has been assigned to variable in_df when it was handed to the operate as an argument. Each the unique df and the in_df contained in the operate level to the identical reminiscence location (numeric worth in parentheses), even when they go by totally different variable names. In the course of the modification of its attributes, the situation of the mutable object stays unchanged. Now all different scopes can see the modifications too — they attain to the identical reminiscence location.

Modification of a mutable object in Python reminiscence.

Truly, since we’ve modified the unique occasion, it’s redundant to return the DataFrame and assign it to the variable. This code has the very same impact:

Modification of a mutable object in Python reminiscence, redundant project eliminated.

Heads-up: the operate now returns None, so watch out to not overwrite the df with None if you happen to do carry out the project: df = modify_df(df).

In distinction, if the article is immutable, it would change the reminiscence location all through the modification identical to within the instance under. Because the pink string can’t be modified (strings are immutable), the inexperienced string is created on high of the outdated one, however as a model new object, claiming a brand new location in reminiscence. The returned string shouldn’t be the identical string, whereas the returned DataFrame was the very same DataFrame.

Modification of an immutable object in Python reminiscence.

The purpose is, mutating DataFrames inside capabilities has a world impact. In case you don’t maintain that in thoughts, it’s possible you’ll:

  • by accident modify or take away a part of your information, considering that the motion is simply going down contained in the operate scope — it isn’t,
  • lose management over what’s added to your DataFrame and when it is added, for instance in nested operate calls.

Output arguments

We’ll repair that drawback later, however right here is one other do not earlier than we cross to do‘s

The design from the earlier part is definitely an anti-pattern known as output argument [1 p.45]. Usually, inputs of a operate can be used to create an output worth. If the only level of passing an argument to a operate is to change it, in order that the enter argument modifications its state, then it’s difficult our intuitions. Such habits known as aspect impact [1 p.44] of a operate and people ought to be properly documented and minimized as a result of they drive the programmer to recollect the issues that go within the background, subsequently making the script error-prone.

Once we learn a operate, we’re used to the concept of knowledge getting into to the operate by means of arguments and out by means of the return worth. We don’t normally count on info to be going out by means of the arguments. [1 p.41]

Issues get even worse if the operate has a double accountability: to change the enter and to return an output. Take into account this operate:

def find_max_name_length(df: pd.DataFrame) -> int:
df["name_len"] = df["name"].str.len() # aspect impact
return max(df["name_len"])

It does return a price as you’ll count on, but it surely additionally completely modifies the unique DataFrame. The aspect impact takes you abruptly – nothing within the operate signature indicated that our enter information was going to be affected. Within the subsequent step, we’ll see methods to keep away from this type of design.

Scale back modifications

To get rid of the aspect impact, within the code under we’ve created a brand new non permanent variable as an alternative of modifying the unique DataFrame. The notation lengths: pd.Collection signifies the datatype of the variable.

def find_max_name_length(df: pd.DataFrame) -> int:
lengths: pd.Collection = df["name"].str.len()
return max(lengths)

This operate design is healthier in that it encapsulates the intermediate state as an alternative of manufacturing a aspect impact.

One other heads-up: please be aware of the variations between deep and shallow copy [6] of parts from the DataFrame. Within the instance above we’ve modified every ingredient of the unique df["name"] Collection, so the outdated DataFrame and the brand new variable don’t have any shared parts. Nevertheless, if you happen to instantly assign one of many unique columns to a brand new variable, the underlying parts nonetheless have the identical references in reminiscence. See the examples:

df = pd.DataFrame({"title": ["bert", "albert"]})

collection = df["name"] # shallow copy
collection[0] = "roberta" # <-- this modifications the unique DataFrame

collection = df["name"].copy(deep=True)
collection[0] = "roberta" # <-- this doesn't change the unique DataFrame

collection = df["name"].str.title() # not a duplicate by any means
collection[0] = "roberta" # <-- this doesn't change the unique DataFrame

You may print out the DataFrame after every step to watch the impact. Keep in mind that making a deep copy will allocate new reminiscence, so it’s good to mirror whether or not your script must be memory-efficient.

Group related operations

Perhaps for no matter cause you wish to retailer the results of that size computation. It’s nonetheless not a good suggestion to append it to the DataFrame contained in the operate due to the aspect impact breach in addition to the buildup of a number of tasks inside a single operate.

I just like the One Stage of Abstraction per Operate rule that claims:

We have to make it possible for the statements inside our operate are all on the similar degree of abstraction.

Mixing ranges of abstraction inside a operate is at all times complicated. Readers might not be capable to inform whether or not a selected expression is an important idea or a element. [1 p.36]

Additionally let’s make use of the Single accountability precept [1 p.138] from OOP, though we’re not specializing in object-oriented code proper now.

Why not put together your information beforehand? Let’s break up information preparation and the precise computation in separate capabilities.:

def create_name_len_col(collection: pd.Collection) -> pd.Collection:
return collection.str.len()

def find_max_element(assortment: Assortment) -> int:
return max(assortment) if len(assortment) else 0

df = pd.DataFrame({"title": ["bert", "albert"]})
df["name_len"] = create_name_len_col(df.title)
max_name_len = find_max_element(df.name_len)

The person activity of making the name_len column has been outsourced to a different operate. It doesn’t modify the unique DataFrame and it performs one activity at a time. Later we retrieve the max ingredient by passing the brand new column to a different devoted operate. Discover how the aggregating operate is generic for Collections.

Let’s brush the code up with the next steps:

  • We might use concat operate and extract it to a separate operate known as prepare_data, which might group all information preparation steps in a single place,
  • We might additionally make use of the apply methodology and work on particular person texts as an alternative of Collection of texts,
  • Let’s keep in mind to make use of shallow vs. deep copy, relying on whether or not the unique information ought to or shouldn’t be modified:
def compute_length(phrase: str) -> int:
return len(phrase)

def prepare_data(df: pd.DataFrame) -> pd.DataFrame:
return pd.concat([
df.copy(deep=True), # deep copy
], axis=1)


The best way we’ve break up the code actually makes it straightforward to return to the script later, take the whole operate and reuse it in one other script. We like that!

There may be yet one more factor we will do to extend the extent of reusability: cross column names as parameters to capabilities. The refactoring goes slightly bit excessive, however generally it pays for the sake of flexibility or reusability.

def create_name_len_col(df: pd.DataFrame, orig_col: str, target_col: str) -> pd.Collection:
return df[orig_col].str.len().rename(target_col)

name_label, name_len_label = "title", "name_len"
create_name_len_col(df, name_label, name_len_label)
], axis=1)


Did you ever determine that your preprocessing was defective after weeks of experiments on the preprocessed dataset? No? Fortunate you. I really needed to repeat a batch of experiments due to damaged annotations, which might have been averted if I had examined simply a few fundamental capabilities.

Vital scripts ought to be examined [1 p.121, 7]. Even when the script is only a helper, I now attempt to check not less than the essential, most low-level capabilities. Let’s revisit the steps that we produced from the beginning:

1. I’m not joyful to even consider testing this, it’s very redundant and we’ve paved over the aspect impact. It additionally checks a bunch of various options: the computation of title size and the aggregation of end result for the max ingredient. Plus it fails, did you see that coming?

def find_max_name_length(df: pd.DataFrame) -> int:
df["name_len"] = df["name"].str.len() # aspect impact
return max(df["name_len"])

@pytest.mark.parametrize("df, end result", [
(pd.DataFrame({"name": []}), 0), # oops, this fails!
(pd.DataFrame({"title": ["bert"]}), 4),
(pd.DataFrame({"title": ["bert", "roberta"]}), 7),
def test_find_max_name_length(df: pd.DataFrame, end result: int):
assert find_max_name_length(df) == end result

2. That is significantly better — we’ve centered on one single activity, so the check is less complicated. We additionally don’t must fixate on column names like we did earlier than. Nevertheless, I feel that the format of the information will get in the best way of verifying the correctness of the computation.

def create_name_len_col(collection: pd.Collection) -> pd.Collection:
return collection.str.len()

@pytest.mark.parametrize("series1, series2", [
(pd.Series([]), pd.Collection([])),
(pd.Collection(["bert"]), pd.Collection([4])),
(pd.Collection(["bert", "roberta"]), pd.Collection([4, 7]))
def test_create_name_len_col(series1: pd.Collection, series2: pd.Collection):
pd.testing.assert_series_equal(create_name_len_col(series1), series2, check_dtype=False)

3. Right here we’ve cleaned up the desk. We check the computation operate inside out, leaving the pandas overlay behind. It’s simpler to provide you with edge circumstances whenever you concentrate on one factor at a time. I discovered that I’d like to check for None values that will seem within the DataFrame and I ultimately had to enhance my operate for that check to cross. A bug caught!

def compute_length(phrase: Non-obligatory[str]) -> int:
return len(phrase) if phrase else 0

@pytest.mark.parametrize("phrase, size", [
("", 0),
("bert", 4),
(None, 0)
def test_compute_length(phrase: str, size: int):
assert compute_length(phrase) == size

4. We’re solely lacking the check for find_max_element:

def find_max_element(assortment: Assortment) -> int:
return max(assortment) if len(assortment) else 0

@pytest.mark.parametrize("assortment, end result", [
([], 0),
([4], 4),
([4, 7], 7),
(pd.Collection([4, 7]), 7),
def test_find_max_element(assortment: Assortment, end result: int):
assert find_max_element(assortment) == end result

One further good thing about unit testing that I always remember to say is that it’s a manner of documenting your code, as somebody who doesn’t realize it (like you from the longer term) can simply determine the inputs and anticipated outputs, together with edge circumstances, simply by wanting on the checks. Double achieve!

These are some tips I discovered helpful whereas coding and reviewing different individuals’s code. I’m removed from telling you that one or one other manner of coding is the one right one — you are taking what you need from it, you resolve whether or not you want a fast scratch or a extremely polished and examined codebase. I hope this thought piece helps you construction your scripts so that you simply’re happier with them and extra assured about their infallibility.

In case you favored this text, I’d like to learn about it. Blissful coding!


There’s nobody and solely right manner of coding, however listed below are some inspirations for scripting with pandas:


– don’t mutate your DataFrame an excessive amount of inside capabilities, as a result of it’s possible you’ll lose management over what and the place will get appended/faraway from it,

– don’t write strategies that mutate a DataFrame and return nothing as a result of that is complicated.


– create new objects as an alternative of modifying the supply DataFrame and keep in mind to make a deep copy when wanted,

– carry out solely similar-level operations inside a single operate,

– design capabilities for flexibility and reusability,

– check your capabilities as a result of this helps you design cleaner code, safe towards bugs and edge circumstances and doc it at no cost.

The graphs have been created by me utilizing Miro. The quilt picture was additionally created by me utilizing the Titanic dataset and GIMP (smudge impact).

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