The rich ecosystem of Python libraries is one of the reasons why Python is the standard choice for data science.

As the ecosystem is so vast, it can be tough to know whele to start.

We suggest starting your data science journey with two Python libraries

**pandas**for loading, transforming and saving data.**matplotlib**for visualizing data.

Together these two libraries offer enough to start doing meaningful data work in Python. Many Excel based workflows can be completely replaced and improved with these two libraries.

In this article we will work with a simple `cities`

dataset - a tabular dataset with three rows and three columns:

pandas is a Python library for working with tabular data - data with rows and columns.

If you have worked with Excel, you have already worked with tabular data.

You can install pandas with `pip`

, a Python package manager:

```
$ pip install pandas
```

You can also find a notebook with all the code we develop in this article on Github or on Google Colab.

Pandas is used by importing the `pandas`

package, commonly aliased to `pd`

:

```
import pandas as pd
```

At the core of pandas is the DataFrame (a `pd.DataFrame`

Python object), composed of three parts:

**index**- 1-dimensional row labels,**columns**- 1-dimensional column labels,**values**- 2-dimensional data.

Both the index and columns are labels - they tell us what a row or column represents, such as the `population`

column label.

One way to create a DataFrame is from a Python dictionary, with:

- the column names as dictionary keys,
- the data as the dictionary values.

Below we create a `cities`

DataFrame from a dictionary:

```
import pandas as pd
pd.DataFrame({
'city': ['auckland', 'berlin', 'london'],
'population': [1.6, 3.6, 8.9],
'hemisphere': ['south', 'north', 'north']
})
"""
city population hemisphere
0 auckland 1.6 south
1 berlin 3.6 north
2 london 8.9 north
"""
```

Being able to construct DataFrames from Python objects such as dictionaries and lists is useful for constructing data for unit tests.

A more common way to create a DataFrame is from a file - commonly a CSV file. This allows us to work with data saved on files on our local machine.

Pandas handles this with `pd.read_csv`

, which reads data from a CSV file on our local computer. This same function can also be used to read data from the Internet.

Below we read in our `cities`

dataset using `pd.read_csv`

to read from a Github URI:

```
import pandas as pd
data = pd.read_csv(
'https://raw.githubusercontent.com/ADGEfficiency/data-science-south-data/main/cities/cities.csv'
)
"""
city population hemisphere
0 auckland 1.6 south
1 berlin 3.6 north
2 london 8.9 north
"""
```

This is the way we will load our `cities`

dataset in rest of this post - reading directly from a public URI (such as a URL).

We can access the three parts of our DataFrame as attributes of an initialized `pd.Dataframe`

object:

```
data.index
# RangeIndex(start=0, stop=3, step=1)
data.columns
# Index(['city', 'population', 'hemisphere'], dtype='object')
data.values
"""
[['auckland' 1.6 'south']
['berlin' 3.6 'north']
['london' 8.9 'north']]
"""
```

The index pandas created for us is just a range index (also called an integer index) - rows are labelled with a sequence of integers (`[0, 1, 2]`

):

```
data.index
# RangeIndex(start=0, stop=3, step=1)
data
"""
city population hemisphere
0 auckland 1.6 south
1 berlin 3.6 north
2 london 8.9 north
"""
```

We can replace this with more meaningful index using `set_index`

, turning the `city`

column into the index:

```
data.set_index('city')
"""
population hemisphere
city
auckland 1.6 south
berlin 3.6 north
london 8.9 north
"""
```

We have lost our original integer index of `[0, 1, 2]`

and gained an index of `city`

- great!

A basic operation in data analysis is selection - to select rows and columns. There are two ways to do this in pandas - `loc`

and `iloc`

.

We want to select rows or columns in two ways:

- a label using
`loc`

, - a position using
`iloc`

.

Both require us to specify both the row and columns (by either label or position) - using `:`

to select the entire row or column.

`loc`

uses labels`loc`

selects **based on the label** of the row and column.

`loc`

allows us to use the labels of the index and columns - we use it to select data based on labels:

```
# select the berlin row, all columns
data.loc['berlin', :]
# select all rows, second column
data.loc[:, 'population']
```

`iloc`

uses position`iloc`

selects **based on the integer position** of the row and column.

`iloc`

allows us to use the position of rows and columns - we use it to select data based on position:

```
# select the first row, all columns
data.iloc[0, :]
# select all rows, second column
data.iloc[:, 1]
```

Selecting based on position is very useful when your data is sorted.

`iloc`

is why a range index isn't that useful in pandas - we can always use `iloc`

to select data based on it's position.

Now we have been introduced to `loc`

& `iloc`

, let's use them to answer two questions about our `cities`

dataset.

We can answer this using `loc`

, selecting the `auckland`

row and the `population`

column:

```
data.loc['auckland', 'population']
# 1.6
```

We can answer this using `iloc`

to select the first row with `0`

and `loc`

to select the `population`

column:

```
data.iloc[0].loc['hemisphere']
# north
```

Another basic operation in data analysis is filtering - selecting rows or columns based on conditional logic (if statements, equalities like `==`

and inequalities like `>`

or `<`

).

We can filter in pandas with a boolean mask, which can be created with a conditional statement:

```
import pandas as pd
data = pd.read_csv('https://raw.githubusercontent.com/ADGEfficiency/data-science-south-data/main/cities/cities.csv')
# create our boolean mask
# with the conditional 'population < 2.0'
# aka population less than 2.0
mask = data.loc[:, 'population'] < 2.0
"""
city
auckland True
berlin False
london False
Name: population, dtype: bool
"""
```

The boolean mask is an array of either `True`

or `False`

- here indicating `True`

if the population of the city is less than 2.

We can use our boolean mask to filter our dataset with `loc`

- `loc`

understands how to use a boolean mask:

```
subset = data.loc[mask, :]
"""
population hemisphere
city
auckland 1.6 south
"""
```

`loc`

or `iloc`

?The power of using boolean masks is that we can select many rows at once.

Below we create a boolean mask based on the hemisphere of the city - then using this mask to select two rows:

```
mask = data.loc[:, 'hemisphere'] == 'north'
"""
city
auckland False
berlin True
london True
Name: hemisphere, dtype: bool
"""
subset = data.loc[mask, :]
"""
population hemisphere
city
berlin 3.6 north
london 8.9 north
"""
```

The final data analysis operation we will look at is aggregation.

In pandas aggregation can be done by grouping - using the `groupby`

method on a DataFrame.

In pandas aggregation happens in two steps:

- creating groups based on a column, such as by
`hemisphere`

, - applying an aggregation function to each group, such as counting or averaging.

Aggregation allows us to estimate statistics, lets use it to answer a few more questions.

We can answer this with our two step workflow:

`groupby('hemisphere')`

to group by hemisphere,`mean()`

to calculate the average for each of our hemisphere groups (`north`

and`south`

).

```
import pandas as pd
data = pd.read_csv('https://raw.githubusercontent.com/ADGEfficiency/data-science-south-data/main/cities/cities.csv').set_index('city')
data.groupby('hemisphere').mean()
"""
population
hemisphere
north 6.25
south 1.60
"""
```

Our two steps to answer this question are:

- grouping by
`hemisphere`

, - aggregating with a
`sum`

.

```
data.loc[:, ['population', 'hemisphere']].groupby('hemisphere').sum()
"""
population
hemisphere
north 12.5
south 1.6
"""
```

The last step of our process is going to be to save our data to our harddrive, in Excel friendly CSV format:

```
data.to_csv('groups.csv')
```

That is it for our first look at pandas - all the code we looked at above is given below in full:

```
import pandas as pd
# read dataset from github url
data = pd.read_csv('https://raw.githubusercontent.com/ADGEfficiency/data-science-south-data/main/cities/cities.csv').set_index('city')
# save cities dataset to local machine
data.to_csv('cities.csv')
# select the berlin row, all columns
data.loc['berlin', :]
# select all rows, second colun
data.loc[:, 'population']
# select the first row, all columns
data.iloc[0, :]
# select all rows, second column
data.iloc[:, 1]
# What is the population of Auckland?
data.loc['auckland', 'population']
# Which hemisphere is our first city in?
data.iloc[0].loc['hemisphere']
# select population less than 2
mask = data.loc[:, 'population'] < 2.0
subset = data.loc[mask, :]
# northern hemisphere countries
mask = data.loc[:, 'hemisphere'] == 'north'
subset = data.loc[mask, :]
# average population in each hemisphere
data.groupby('hemisphere').mean()
# total population in each hemisphere
data.loc[:, ['population', 'hemisphere']].groupby('hemisphere').sum()
# save to csv file on local computer
data.to_csv('groups.csv')
```

matplotlib is a Python library for creating visualizations of data.

One of the challenges with learning and using matplotlib is that it offers multiple ways to plot data. Mastering matplotlib requires understanding which API is best for your data and workflow.

We will use a single matplotlib workflow - the one we use the most ourselves. It offers the flexibility to plot multiple charts in the same figure, and integrates nicely with pandas.

You can install matplotlib with `pip`

, a Python package manager:

```
$ pip install matplotlib
```

You can also find a notebook with all the code on Github or on Google Colab.

Matplotlib is used by importing the `matplotlib`

package, commonly aliasing the module `pyplot`

as `plt`

:

```
import matplotlib.pyplot as plt
```

The core components in matplotlib are the figure and axes.

One figure can have many axes:

A common point of confusion is between axis (such as the x or y axis) and a matplotlib axes - they are not the same thing!

One figure can have multiple axes - each axes is a separate plot or chart. Each of these axes has it's own x-axis and y-axis (one of each).

We can create these two objects using `plt.subplots`

- creating one figure with two axes:

```
import matplotlib.pyplot as plt
fig, axes = plt.subplots(ncols=2)
```

All we've done above is create a figure with two axes - both empty.

Next we load our data with pandas, and create a plot on our first axes:

```
import pandas as pd
# run a simple data pipeline - load data from CSV with pandas
data = pd.read_csv('https://raw.githubusercontent.com/ADGEfficiency/data-science-south-data/main/cities/cities.csv').set_index('city')
# access first axes and plot a line
data.plot('population', ax=axes[0], kind='bar')
```

Automatically makes labels for the x and y axis - quite nice!

Our second axes is still empty - we can plot something on it by passing `ax=axes[1]`

into another `plot`

call on our DataFrame:

```
# access first axes and plot a scatter plot
data.plot('land-area', 'population', ax=axes[1], kind='scatter')
```

Now we can see two visualizations of our data - a bar chart and a scatter plot.

The final step in our pipeline is to save our figure to a PNG file on our local machine:

```
fig.savefig('cities.png')
```

The full code for our visualization pipeline is below:

```
import matplotlib.pyplot as plt
import pandas as pd
# create one figure with two axes
fig, axes = plt.subplots(nrows=2)
# run a simple data pipeline
data = pd.read_csv('https://raw.githubusercontent.com/ADGEfficiency/data-science-south-data/main/cities/cities.csv').set_index('city')
# access first axes and plot a line
data.plot(y='population', ax=axes[0], kind='bar')
# access first axes and plot a scatter plot
data.plot('land-area', 'population', ax=axes[1], kind='scatter')
# small trick to get x-axis labels to play nice
plt.tight_layout()
# save the figure as a png file
fig.savefig('cities.png')
```

There is much more to learn with both of these libraries.

Suggested next steps for pandas:

- learn about
`pd.Series`

- another pandas object used to store data, - learn how to save data to Parquet format (and why),
- see how we can use
`.agg`

for different aggregations with a single groupby.

Suggested next steps for matplotlib:

- learn to plot data using
`plt.plot`

, - learn to plot data using
`ax.plot`

, - check out seaborn - a library that extends matplotlib with charts like the pairplot.