3.2.3. Common Functions/Methods

Note

Some pandas methods work on a dataframe, like df.assign(feet=df['height']//12). These are methods that are altering a dataframe, and you use them like this: <dfname>.<method>(<arguments>).

Note: Delete the < and > after you type in the dataframe name, method, and arguments. Those are just indicating the text inside them is a placeholder.

Some pandas methods are called on the pandas module itself (e.g. pd.merge). These are methods that are doing tasks outside a dataframe (like loading or merging datasets), and you use them like this: pd.<method>(<arguments>)

Remember the SHIFT+TAB trick to see function help!

Type import pandas as pd then run that to load pandas. Then type pd.merge( like you want to merge to dataframes, except you don’t remember the arguments to use. So type SHIFT+TAB to see the function’s documentation!

Loading and saving data

Function

Pandas method

Example (see official syntax for more)

loading data

read_csv, read_dta, etc

pd.read_csv('wine.csv')

saving data

to_csv, to_dta, etc

pd.to_csv('wine.csv')
Manipulating data ⭐

Warning

df.assign(feet=df['height']//12) will not add a “feet” variable to df permanently. This is true of almost all dataframe methods (e.g. filter, rename, …). If you want to save the new variable, you need to type df = df.assign(feet=df['height']//12). See the next page for more.

Note

Remember: replace df below with the name of the dataframe you’re working on!

Function

Pandas method

Examples (see official syntax for more)

new variables or replace existing

assign

df.assign(feet=df['height']//12)
df['feet'] = df['height']//12

filter or get subset of observations
or, “drop rows”

⭐ query / loc / iloc

df.query('height > 68')
df.loc[df['gender']=='F']
df.iloc[1:]

get subset of columns

filter

df.filter(['height','weight'])
df[['height','weight']]

rename columns

rename

df.rename(columns={"height": "how tall"})

sort

sort_values

df.sort_values(['gender','weight'])

do an operation on groups of observations

groupby ⭐

df.groupby(['gender']) , see common tasks for more. But if you think “I’d like to do a “for-loop” on this dataframe… the answer is usually groupby

summary stats

agg / pivot_table

df.agg({'height':[max,min,np.mean]})

df.pivot_table(index='age', columns='age', values='weight')

summary stats on groups

agg / pivot_table

df.groupby(['gender'])
.agg({'height':[max,min,np.mean]})

df.pivot_table(index='age', columns='age', values='weight'

create a variable based on its group

agg+transform

df.groupby(['industry','year'])['leverage'].mean().transform()
will add industry average leverage to your dataset for each firm

delete column

drop

df.drop(columns=['gender'])

use non-pd function on df

pipe

df.pipe((sns.lineplot,data),x=x,y=y)

combine dataframes

merge

pd.merge(df1,df2)

change time frequency of data

resample

df.resample('Y').mean()

window/rolling calculations

window

df['vol_5yr']= df.groupby('firm').rolling(36).var('ret').transform() will add 36 period volatility for each firm
Reshaping data and changing index

Function

Pandas method

Example (see official syntax for more)

convert wide to long/tall (“stack!”)

stack

df.stack(), see common tasks for usage examples and The Shape of Data for explanation of wide/tall

melt is another option and is a special case of stack

convert long/tall to wide (“unstack!”)

unstack

df.unstack(), see common tasks for usage examples and The Shape of Data for explanation of wide/tall

pivot_table is another option and is a special case of unstack

turn a variable column into the index

set_index

df.set_index('SSN')

turn the index into a variable

reset_index

df.reset_index()

Note: The new index will just be the row numbers.
Statistical operations

  1. These functions can be called for a variable “col1” in this form: <dfname>['columnname'].<function>() or for all numerical columns at once using <dfname>.<function>().

  2. These functions work within groups. ⭐

Function

Description

count

Number of non-null observations

sum

Sum of values

mean

Mean of values

mad

Mean absolute deviation

median

Arithmetic median of values

min

Minimum

max

Maximum

mode

Mode

abs

Absolute Value

prod

Product of values

std

Unbiased standard deviation

var

Unbiased variance

sem

Unbiased standard error of the mean

skew

Unbiased skewness (3rd moment)

kurt

Unbiased kurtosis (4th moment)

quantile

Sample quantile (value at %)

cumsum

Cumulative sum

cumprod

Cumulative product

cummax

Cumulative maximum

cummin

Cumulative minimum

nunique

How many unique values?

value_counts

How many of each unique value are there?
3.2.2. Pandas Vocab and the Shape of Data 3.2.4. Temporary vs. Permanent Methods