Tools - pandas
Supplementary material on pandas
First, let's import pandas. People usually import it as pd:
import pandas as pd
Series objects
The pandas library contains these useful data structures:
Seriesobjects, that we will discuss now. ASeriesobject is 1D array, similar to a column in a spreadsheet (with a column name and row labels).DataFrameobjects. This is a 2D table, similar to a spreadsheet (with column names and row labels).Panelobjects. You can see aPanelas a dictionary ofDataFrames. These are less used, so we will not discuss them here.
s = pd.Series([2,-1,3,5])
s
import numpy as np
np.exp(s)
Arithmetic operations on Series are also possible, and they apply elementwise, just like for ndarrays:
s + [1000,2000,3000,4000]
Similar to NumPy, if you add a single number to a Series, that number is added to all items in the Series. This is called broadcasting:
s + 1000
The same is true for all binary operations such as * or /, and even conditional operations:
s < 0
s2 = pd.Series([68, 83, 112, 68], index=["alice", "bob", "charles", "darwin"])
s2
You can then use the Series just like a dict:
s2["bob"]
You can still access the items by integer location, like in a regular array:
s2[1]
To make it clear when you are accessing by label or by integer location, it is recommended to always use the loc attribute when accessing by label, and the iloc attribute when accessing by integer location:
s2.loc["bob"]
s2.iloc[1]
Slicing a Series also slices the index labels:
s2.iloc[1:3]
This can lead to unexpected results when using the default numeric labels, so be careful:
surprise = pd.Series([1000, 1001, 1002, 1003])
surprise
surprise_slice = surprise[2:]
surprise_slice
Oh look! The first element has index label 2. The element with index label 0 is absent from the slice:
try:
surprise_slice[0]
except KeyError as e:
print("Key error:", e)
But remember that you can access elements by integer location using the iloc attribute. This illustrates another reason why it's always better to use loc and iloc to access Series objects:
surprise_slice.iloc[0]
weights = {"alice": 68, "bob": 83, "colin": 86, "darwin": 68}
s3 = pd.Series(weights)
s3
You can control which elements you want to include in the Series and in what order by explicitly specifying the desired index:
s4 = pd.Series(weights, index = ["colin", "alice"])
s4
print(s2.keys())
print(s3.keys())
s2 + s3
The resulting Series contains the union of index labels from s2 and s3. Since "colin" is missing from s2 and "charles" is missing from s3, these items have a NaN result value. (ie. Not-a-Number means missing).
Automatic alignment is very handy when working with data that may come from various sources with varying structure and missing items. But if you forget to set the right index labels, you can have surprising results:
s5 = pd.Series([1000,1000,1000,1000])
print("s2 =", s2.values)
print("s5 =", s5.values)
s2 + s5
Pandas could not align the Series, since their labels do not match at all, hence the full NaN result.
meaning = pd.Series(42, ["life", "universe", "everything"])
meaning
s6 = pd.Series([83, 68], index=["bob", "alice"], name="weights")
s6
Plotting a Series
Pandas makes it easy to plot Series data using matplotlib (for more details on matplotlib, check out the matplotlib tutorial). Just import matplotlib and call the plot() method:
%matplotlib inline
import matplotlib.pyplot as plt
temperatures = [4.4,5.1,6.1,6.2,6.1,6.1,5.7,5.2,4.7,4.1,3.9,3.5]
s7 = pd.Series(temperatures, name="Temperature")
s7.plot()
plt.show()
There are many options for plotting your data. It is not necessary to list them all here: if you need a particular type of plot (histograms, pie charts, etc.), just look for it in the excellent Visualization section of pandas' documentation, and look at the example code.
Handling time
Many datasets have timestamps, and pandas is awesome at manipulating such data:
- it can represent periods (such as 2016Q3) and frequencies (such as "monthly"),
- it can convert periods to actual timestamps, and vice versa,
- it can resample data and aggregate values any way you like,
- it can handle timezones.
Time range
Let's start by creating a time series using pd.date_range(). This returns a DatetimeIndex containing one datetime per hour for 12 hours starting on October 29th 2016 at 5:30pm.
dates = pd.date_range('2016/10/29 5:30pm', periods=12, freq='H')
dates
This DatetimeIndex may be used as an index in a Series:
temp_series = pd.Series(temperatures, dates)
temp_series
Let's plot this series:
temp_series.plot(kind="bar")
plt.grid(True)
plt.show()
temp_series_freq_2H = temp_series.resample("2H")
temp_series_freq_2H
The resampling operation is actually a deferred operation, which is why we did not get a Series object, but a DatetimeIndexResampler object instead. To actually perform the resampling operation, we can simply call the mean() method: Pandas will compute the mean of every pair of consecutive hours:
temp_series_freq_2H = temp_series_freq_2H.mean()
Let's plot the result:
temp_series_freq_2H.plot(kind="bar")
plt.show()
Note how the values have automatically been aggregated into 2-hour periods. If we look at the 6-8pm period, for example, we had a value of 5.1 at 6:30pm, and 6.1 at 7:30pm. After resampling, we just have one value of 5.6, which is the mean of 5.1 and 6.1. Rather than computing the mean, we could have used any other aggregation function, for example we can decide to keep the minimum value of each period:
temp_series_freq_2H = temp_series.resample("2H").min()
temp_series_freq_2H
Or, equivalently, we could use the apply() method instead:
temp_series_freq_2H = temp_series.resample("2H").apply(np.min)
temp_series_freq_2H
temp_series_freq_15min = temp_series.resample("15Min").mean()
temp_series_freq_15min.head(n=10) # `head` displays the top n values
One solution is to fill the gaps by interpolating. We just call the interpolate() method. The default is to use linear interpolation, but we can also select another method, such as cubic interpolation:
temp_series_freq_15min = temp_series.resample("15Min").interpolate(method="cubic")
temp_series_freq_15min.head(n=10)
temp_series.plot(label="Period: 1 hour")
temp_series_freq_15min.plot(label="Period: 15 minutes")
plt.legend()
plt.show()
temp_series_ny = temp_series.tz_localize("America/New_York")
temp_series_ny
Note that -04:00 is now appended to all the datetimes. This means that these datetimes refer to UTC - 4 hours.
We can convert these datetimes to Paris time like this:
temp_series_paris = temp_series_ny.tz_convert("Europe/Paris")
temp_series_paris
You may have noticed that the UTC offset changes from +02:00 to +01:00: this is because France switches to winter time at 3am that particular night (time goes back to 2am). Notice that 2:30am occurs twice! Let's go back to a naive representation (if you log some data hourly using local time, without storing the timezone, you might get something like this):
temp_series_paris_naive = temp_series_paris.tz_localize(None)
temp_series_paris_naive
Now 02:30 is really ambiguous. If we try to localize these naive datetimes to the Paris timezone, we get an error:
try:
temp_series_paris_naive.tz_localize("Europe/Paris")
except Exception as e:
print(type(e))
print(e)
Fortunately using the ambiguous argument we can tell pandas to infer the right DST (Daylight Saving Time) based on the order of the ambiguous timestamps:
temp_series_paris_naive.tz_localize("Europe/Paris", ambiguous="infer")
quarters = pd.period_range('2016Q1', periods=8, freq='Q')
quarters
Adding a number N to a PeriodIndex shifts the periods by N times the PeriodIndex's frequency:
quarters + 3
The asfreq() method lets us change the frequency of the PeriodIndex. All periods are lengthened or shortened accordingly. For example, let's convert all the quarterly periods to monthly periods (zooming in):
quarters.asfreq("M")
By default, the asfreq zooms on the end of each period. We can tell it to zoom on the start of each period instead:
quarters.asfreq("M", how="start")
And we can zoom out:
quarters.asfreq("A")
Of course we can create a Series with a PeriodIndex:
quarterly_revenue = pd.Series([300, 320, 290, 390, 320, 360, 310, 410], index = quarters)
quarterly_revenue
quarterly_revenue.plot(kind="line")
plt.show()
We can convert periods to timestamps by calling to_timestamp. By default this will give us the first day of each period, but by setting how and freq, we can get the last hour of each period:
last_hours = quarterly_revenue.to_timestamp(how="end", freq="H")
last_hours
And back to periods by calling to_period:
last_hours.to_period()
Pandas also provides many other time-related functions that we recommend you check out in the documentation. To whet your appetite, here is one way to get the last business day of each month in 2016, at 9am:
months_2016 = pd.period_range("2016", periods=12, freq="M")
one_day_after_last_days = months_2016.asfreq("D") + 1
last_bdays = one_day_after_last_days.to_timestamp() - pd.tseries.offsets.BDay()
last_bdays.to_period("H") + 9
DataFrame objects
A DataFrame object represents a spreadsheet, with cell values, column names and row index labels. You can define expressions to compute columns based on other columns, create pivot-tables, group rows, draw graphs, etc. You can see DataFrames as dictionaries of Series.
Creating a DataFrame
You can create a DataFrame by passing a dictionary of Series objects:
people_dict = {
"weight": pd.Series([68, 83, 112], index=["alice", "bob", "charles"]),
"birthyear": pd.Series([1984, 1985, 1992], index=["bob", "alice", "charles"], name="year"),
"children": pd.Series([0, 3], index=["charles", "bob"]),
"hobby": pd.Series(["Biking", "Dancing"], index=["alice", "bob"]),
}
people = pd.DataFrame(people_dict)
people
A few things to note:
- the
Serieswere automatically aligned based on their index, - missing values are represented as
NaN, Seriesnames are ignored (the name"year"was dropped),DataFrames are displayed nicely in Jupyter notebooks, woohoo!
You can access columns pretty much as you would expect. They are returned as Series objects:
people["birthyear"]
You can also get multiple columns at once:
people[["birthyear", "hobby"]]
If you pass a list of columns and/or index row labels to the DataFrame constructor, it will guarantee that these columns and/or rows will exist, in that order, and no other column/row will exist. For example:
d2 = pd.DataFrame(
people_dict,
columns=["birthyear", "weight", "height"],
index=["bob", "alice", "eugene"]
)
d2
Another convenient way to create a DataFrame is to pass all the values to the constructor as an ndarray, or a list of lists, and specify the column names and row index labels separately:
values = [
[1985, np.nan, "Biking", 68],
[1984, 3, "Dancing", 83],
[1992, 0, np.nan, 112]
]
d3 = pd.DataFrame(
values,
columns=["birthyear", "children", "hobby", "weight"],
index=["alice", "bob", "charles"]
)
d3
To specify missing values, you can either use np.nan or NumPy's masked arrays:
masked_array = np.ma.asarray(values, dtype=np.object)
masked_array[(0, 2), (1, 2)] = np.ma.masked
d3 = pd.DataFrame(
masked_array,
columns=["birthyear", "children", "hobby", "weight"],
index=["alice", "bob", "charles"]
)
d3
Instead of an ndarray, you can also pass a DataFrame object:
d4 = pd.DataFrame(
d3,
columns=["hobby", "children"],
index=["alice", "bob"]
)
d4
It is also possible to create a DataFrame with a dictionary (or list) of dictionaries (or list):
people = pd.DataFrame({
"birthyear": {"alice":1985, "bob": 1984, "charles": 1992},
"hobby": {"alice":"Biking", "bob": "Dancing"},
"weight": {"alice":68, "bob": 83, "charles": 112},
"children": {"bob": 3, "charles": 0}
})
people
d5 = pd.DataFrame(
{
("public", "birthyear"):
{("Paris","alice"):1985, ("Paris","bob"): 1984, ("London","charles"): 1992},
("public", "hobby"):
{("Paris","alice"):"Biking", ("Paris","bob"): "Dancing"},
("private", "weight"):
{("Paris","alice"):68, ("Paris","bob"): 83, ("London","charles"): 112},
("private", "children"):
{("Paris", "alice"):np.nan, ("Paris","bob"): 3, ("London","charles"): 0}
}
)
d5
You can now get a DataFrame containing all the "public" columns very simply:
d5["public"]
d5["public", "hobby"] # Same result as d5["public"]["hobby"]
d5
There are two levels of columns, and two levels of indices. We can drop a column level by calling droplevel() (the same goes for indices):
d5.columns = d5.columns.droplevel(level = 0)
d5
d6 = d5.T
d6
d7 = d6.stack()
d7
Note that many NaN values appeared. This makes sense because many new combinations did not exist before (eg. there was no bob in London).
Calling unstack() will do the reverse, once again creating many NaN values.
d8 = d7.unstack()
d8
If we call unstack again, we end up with a Series object:
d9 = d8.unstack()
d9
The stack() and unstack() methods let you select the level to stack/unstack. You can even stack/unstack multiple levels at once:
d10 = d9.unstack(level = (0,1))
d10
people
The loc attribute lets you access rows instead of columns. The result is a Series object in which the DataFrame's column names are mapped to row index labels:
people.loc["charles"]
You can also access rows by integer location using the iloc attribute:
people.iloc[2]
You can also get a slice of rows, and this returns a DataFrame object:
people.iloc[1:3]
Finally, you can pass a boolean array to get the matching rows:
people[np.array([True, False, True])]
This is most useful when combined with boolean expressions:
people[people["birthyear"] < 1990]
people
people["age"] = 2018 - people["birthyear"] # adds a new column "age"
people["over 30"] = people["age"] > 30 # adds another column "over 30"
birthyears = people.pop("birthyear")
del people["children"]
people
birthyears
When you add a new colum, it must have the same number of rows. Missing rows are filled with NaN, and extra rows are ignored:
people["pets"] = pd.Series({"bob": 0, "charles": 5, "eugene":1}) # alice is missing, eugene is ignored
people
When adding a new column, it is added at the end (on the right) by default. You can also insert a column anywhere else using the insert() method:
people.insert(1, "height", [172, 181, 185])
people
people.assign(
body_mass_index = people["weight"] / (people["height"] / 100) ** 2,
has_pets = people["pets"] > 0
)
Note that you cannot access columns created within the same assignment:
try:
people.assign(
body_mass_index = people["weight"] / (people["height"] / 100) ** 2,
overweight = people["body_mass_index"] > 25
)
except KeyError as e:
print("Key error:", e)
The solution is to split this assignment in two consecutive assignments:
d6 = people.assign(body_mass_index = people["weight"] / (people["height"] / 100) ** 2)
d6.assign(overweight = d6["body_mass_index"] > 25)
Having to create a temporary variable d6 is not very convenient. You may want to just chain the assigment calls, but it does not work because the people object is not actually modified by the first assignment:
try:
(people
.assign(body_mass_index = people["weight"] / (people["height"] / 100) ** 2)
.assign(overweight = people["body_mass_index"] > 25)
)
except KeyError as e:
print("Key error:", e)
But fear not, there is a simple solution. You can pass a function to the assign() method (typically a lambda function), and this function will be called with the DataFrame as a parameter:
(people
.assign(body_mass_index = lambda df: df["weight"] / (df["height"] / 100) ** 2)
.assign(overweight = lambda df: df["body_mass_index"] > 25)
)
Problem solved!
people.eval("weight / (height/100) ** 2 > 25")
Assignment expressions are also supported. Let's set inplace=True to directly modify the DataFrame rather than getting a modified copy:
people.eval("body_mass_index = weight / (height/100) ** 2", inplace=True)
people
You can use a local or global variable in an expression by prefixing it with '@':
overweight_threshold = 30
people.eval("overweight = body_mass_index > @overweight_threshold", inplace=True)
people
people.query("age > 30 and pets == 0")
people.sort_index(ascending=False)
Note that sort_index returned a sorted copy of the DataFrame. To modify people directly, we can set the inplace argument to True. Also, we can sort the columns instead of the rows by setting axis=1:
people.sort_index(axis=1, inplace=True)
people
To sort the DataFrame by the values instead of the labels, we can use sort_values and specify the column to sort by:
people.sort_values(by="age", inplace=True)
people
people.plot(kind = "line", x = "body_mass_index", y = ["height", "weight"])
plt.show()
You can pass extra arguments supported by matplotlib's functions. For example, we can create scatterplot and pass it a list of sizes using the s argument of matplotlib's scatter() function:
people.plot(kind = "scatter", x = "height", y = "weight", s=[40, 120, 200])
plt.show()
Again, there are way too many options to list here: the best option is to scroll through the Visualization page in pandas' documentation, find the plot you are interested in and look at the example code.
grades_array = np.array([[8,8,9],[10,9,9],[4, 8, 2], [9, 10, 10]])
grades = pd.DataFrame(grades_array, columns=["sep", "oct", "nov"], index=["alice","bob","charles","darwin"])
grades
You can apply NumPy mathematical functions on a DataFrame: the function is applied to all values:
np.sqrt(grades)
Similarly, adding a single value to a DataFrame will add that value to all elements in the DataFrame. This is called broadcasting:
grades + 1
Of course, the same is true for all other binary operations, including arithmetic (*,/,**...) and conditional (>, ==...) operations:
grades >= 5
Aggregation operations, such as computing the max, the sum or the mean of a DataFrame, apply to each column, and you get back a Series object:
grades.mean()
The all method is also an aggregation operation: it checks whether all values are True or not. Let's see during which months all students got a grade greater than 5:
(grades > 5).all()
Most of these functions take an optional axis parameter which lets you specify along which axis of the DataFrame you want the operation executed. The default is axis=0, meaning that the operation is executed vertically (on each column). You can set axis=1 to execute the operation horizontally (on each row). For example, let's find out which students had all grades greater than 5:
(grades > 5).all(axis = 1)
The any method returns True if any value is True. Let's see who got at least one grade 10:
(grades == 10).any(axis = 1)
If you add a Series object to a DataFrame (or execute any other binary operation), pandas attempts to broadcast the operation to all rows in the DataFrame. This only works if the Series has the same size as the DataFrames rows. For example, let's substract the mean of the DataFrame (a Series object) from the DataFrame:
grades - grades.mean() # equivalent to: grades - [7.75, 8.75, 7.50]
We substracted 7.75 from all September grades, 8.75 from October grades and 7.50 from November grades. It is equivalent to substracting this DataFrame:
pd.DataFrame([[7.75, 8.75, 7.50]]*4, index=grades.index, columns=grades.columns)
If you want to substract the global mean from every grade, here is one way to do it:
grades - grades.values.mean() # substracts the global mean (8.00) from all grades
bonus_array = np.array([[0,np.nan,2],[np.nan,1,0],[0, 1, 0], [3, 3, 0]])
bonus_points = pd.DataFrame(bonus_array, columns=["oct", "nov", "dec"], index=["bob","colin", "darwin", "charles"])
bonus_points
grades + bonus_points
Looks like the addition worked in some cases but way too many elements are now empty. That's because when aligning the DataFrames, some columns and rows were only present on one side, and thus they were considered missing on the other side (NaN). Then adding NaN to a number results in NaN, hence the result.
Handling missing data
Dealing with missing data is a frequent task when working with real life data. Pandas offers a few tools to handle missing data.
Let's try to fix the problem above. For example, we can decide that missing data should result in a zero, instead of NaN. We can replace all NaN values by a any value using the fillna() method:
(grades + bonus_points).fillna(0)
It's a bit unfair that we're setting grades to zero in September, though. Perhaps we should decide that missing grades are missing grades, but missing bonus points should be replaced by zeros:
fixed_bonus_points = bonus_points.fillna(0)
fixed_bonus_points.insert(0, "sep", 0)
fixed_bonus_points.loc["alice"] = 0
grades + fixed_bonus_points
That's much better: although we made up some data, we have not been too unfair.
Another way to handle missing data is to interpolate. Let's look at the bonus_points DataFrame again:
bonus_points
Now let's call the interpolate method. By default, it interpolates vertically (axis=0), so let's tell it to interpolate horizontally (axis=1).
bonus_points.interpolate(axis=1)
Bob had 0 bonus points in October, and 2 in December. When we interpolate for November, we get the mean: 1 bonus point. Colin had 1 bonus point in November, but we do not know how many bonus points he had in September, so we cannot interpolate, this is why there is still a missing value in October after interpolation. To fix this, we can set the September bonus points to 0 before interpolation.
better_bonus_points = bonus_points.copy()
better_bonus_points.insert(0, "sep", 0)
better_bonus_points.loc["alice"] = 0
better_bonus_points = better_bonus_points.interpolate(axis=1)
better_bonus_points
Great, now we have reasonable bonus points everywhere. Let's find out the final grades:
grades + better_bonus_points
It is slightly annoying that the September column ends up on the right. This is because the DataFrames we are adding do not have the exact same columns (the grades DataFrame is missing the "dec" column), so to make things predictable, pandas orders the final columns alphabetically. To fix this, we can simply add the missing column before adding:
grades["dec"] = np.nan
final_grades = grades + better_bonus_points
final_grades
There's not much we can do about December and Colin: it's bad enough that we are making up bonus points, but we can't reasonably make up grades (well I guess some teachers probably do). So let's call the dropna() method to get rid of rows that are full of NaNs:
final_grades_clean = final_grades.dropna(how="all")
final_grades_clean
Now let's remove columns that are full of NaNs by setting the axis argument to 1:
final_grades_clean = final_grades_clean.dropna(axis=1, how="all")
final_grades_clean
final_grades["hobby"] = ["Biking", "Dancing", np.nan, "Dancing", "Biking"]
final_grades
Now let's group data in this DataFrame by hobby:
grouped_grades = final_grades.groupby("hobby")
grouped_grades
We are ready to compute the average grade per hobby:
grouped_grades.mean()
That was easy! Note that the NaN values have simply been skipped when computing the means.
Pivot tables
Pandas supports spreadsheet-like pivot tables that allow quick data summarization. To illustrate this, let's create a simple DataFrame:
bonus_points
more_grades = final_grades_clean.stack().reset_index()
more_grades.columns = ["name", "month", "grade"]
more_grades["bonus"] = [np.nan, np.nan, np.nan, 0, np.nan, 2, 3, 3, 0, 0, 1, 0]
more_grades
Now we can call the pd.pivot_table() function for this DataFrame, asking to group by the name column. By default, pivot_table() computes the mean of each numeric column:
pd.pivot_table(more_grades, index="name")
We can change the aggregation function by setting the aggfunc argument, and we can also specify the list of columns whose values will be aggregated:
pd.pivot_table(more_grades, index="name", values=["grade","bonus"], aggfunc=np.max)
We can also specify the columns to aggregate over horizontally, and request the grand totals for each row and column by setting margins=True:
pd.pivot_table(more_grades, index="name", values="grade", columns="month", margins=True)
Finally, we can specify multiple index or column names, and pandas will create multi-level indices:
pd.pivot_table(more_grades, index=("name", "month"), margins=True)
Overview functions
When dealing with large DataFrames, it is useful to get a quick overview of its content. Pandas offers a few functions for this. First, let's create a large DataFrame with a mix of numeric values, missing values and text values. Notice how Jupyter displays only the corners of the DataFrame:
much_data = np.fromfunction(lambda x,y: (x+y*y)%17*11, (10000, 26))
large_df = pd.DataFrame(much_data, columns=list("ABCDEFGHIJKLMNOPQRSTUVWXYZ"))
large_df[large_df % 16 == 0] = np.nan
large_df.insert(3,"some_text", "Blabla")
large_df
The head() method returns the top 5 rows:
large_df.head()
Of course there's also a tail() function to view the bottom 5 rows. You can pass the number of rows you want:
large_df.tail(n=2)
The info() method prints out a summary of each columns contents:
large_df.info()
Finally, the describe() method gives a nice overview of the main aggregated values over each column:
count: number of non-null (not NaN) valuesmean: mean of non-null valuesstd: standard deviation of non-null valuesmin: minimum of non-null values25%,50%,75%: 25th, 50th and 75th percentile of non-null valuesmax: maximum of non-null values
large_df.describe()
my_df = pd.DataFrame(
[["Biking", 68.5, 1985, np.nan], ["Dancing", 83.1, 1984, 3]],
columns=["hobby","weight","birthyear","children"],
index=["alice", "bob"]
)
my_df
my_df.to_csv("my_df.csv")
my_df.to_html("my_df.html")
my_df.to_json("my_df.json")
Done! Let's take a peek at what was saved:
for filename in ("my_df.csv", "my_df.html", "my_df.json"):
print("#", filename)
with open(filename, "rt") as f:
print(f.read())
print()
Note that the index is saved as the first column (with no name) in a CSV file, as <th> tags in HTML and as keys in JSON.
Saving to other formats works very similarly, but some formats require extra libraries to be installed. For example, saving to Excel requires the openpyxl library:
try:
my_df.to_excel("my_df.xlsx", sheet_name='People')
except ImportError as e:
print(e)
my_df_loaded = pd.read_csv("my_df.csv", index_col=0)
my_df_loaded
As you might guess, there are similar read_json, read_html, read_excel functions as well. We can also read data straight from the Internet. For example, let's load all U.S. cities from simplemaps.com:
us_cities = None
try:
csv_url = "http://simplemaps.com/files/cities.csv"
us_cities = pd.read_csv(csv_url, index_col=0)
us_cities = us_cities.head()
except IOError as e:
print(e)
us_cities
There are more options available, in particular regarding datetime format. Check out the documentation for more details.
city_loc = pd.DataFrame(
[
["CA", "San Francisco", 37.781334, -122.416728],
["NY", "New York", 40.705649, -74.008344],
["FL", "Miami", 25.791100, -80.320733],
["OH", "Cleveland", 41.473508, -81.739791],
["UT", "Salt Lake City", 40.755851, -111.896657]
], columns=["state", "city", "lat", "lng"])
city_loc
city_pop = pd.DataFrame(
[
[808976, "San Francisco", "California"],
[8363710, "New York", "New-York"],
[413201, "Miami", "Florida"],
[2242193, "Houston", "Texas"]
], index=[3,4,5,6], columns=["population", "city", "state"])
city_pop
Now let's join these DataFrames using the merge() function:
pd.merge(left=city_loc, right=city_pop, on="city")
Note that both DataFrames have a column named state, so in the result they got renamed to state_x and state_y.
Also, note that Cleveland, Salt Lake City and Houston were dropped because they don't exist in both DataFrames. This is the equivalent of a SQL INNER JOIN. If you want a FULL OUTER JOIN, where no city gets dropped and NaN values are added, you must specify how="outer":
all_cities = pd.merge(left=city_loc, right=city_pop, on="city", how="outer")
all_cities
Of course LEFT OUTER JOIN is also available by setting how="left": only the cities present in the left DataFrame end up in the result. Similarly, with how="right" only cities in the right DataFrame appear in the result. For example:
pd.merge(left=city_loc, right=city_pop, on="city", how="right")
If the key to join on is actually in one (or both) DataFrame's index, you must use left_index=True and/or right_index=True. If the key column names differ, you must use left_on and right_on. For example:
city_pop2 = city_pop.copy()
city_pop2.columns = ["population", "name", "state"]
pd.merge(left=city_loc, right=city_pop2, left_on="city", right_on="name")
result_concat = pd.concat([city_loc, city_pop])
result_concat
Note that this operation aligned the data horizontally (by columns) but not vertically (by rows). In this example, we end up with multiple rows having the same index (eg. 3). Pandas handles this rather gracefully:
result_concat.loc[3]
Or you can tell pandas to just ignore the index:
pd.concat([city_loc, city_pop], ignore_index=True)
Notice that when a column does not exist in a DataFrame, it acts as if it was filled with NaN values. If we set join="inner", then only columns that exist in both DataFrames are returned:
pd.concat([city_loc, city_pop], join="inner")
You can concatenate DataFrames horizontally instead of vertically by setting axis=1:
pd.concat([city_loc, city_pop], axis=1)
In this case it really does not make much sense because the indices do not align well (eg. Cleveland and San Francisco end up on the same row, because they shared the index label 3). So let's reindex the DataFrames by city name before concatenating:
pd.concat([city_loc.set_index("city"), city_pop.set_index("city")], axis=1)
This looks a lot like a FULL OUTER JOIN, except that the state columns were not renamed to state_x and state_y, and the city column is now the index.
The append() method is a useful shorthand for concatenating DataFrames vertically:
city_loc.append(city_pop)
As always in pandas, the append() method does not actually modify city_loc: it works on a copy and returns the modified copy.
Categories
It is quite frequent to have values that represent categories, for example 1 for female and 2 for male, or "A" for Good, "B" for Average, "C" for Bad. These categorical values can be hard to read and cumbersome to handle, but fortunately pandas makes it easy. To illustrate this, let's take the city_pop DataFrame we created earlier, and add a column that represents a category:
city_eco = city_pop.copy()
city_eco["eco_code"] = [17, 17, 34, 20]
city_eco
Right now the eco_code column is full of apparently meaningless codes. Let's fix that. First, we will create a new categorical column based on the eco_codes:
city_eco["economy"] = city_eco["eco_code"].astype('category')
city_eco["economy"].cat.categories
Now we can give each category a meaningful name:
city_eco["economy"].cat.categories = ["Finance", "Energy", "Tourism"]
city_eco
Note that categorical values are sorted according to their categorical order, not their alphabetical order:
city_eco.sort_values(by="economy", ascending=False)
What next?
As you probably noticed by now, pandas is quite a large library with many features. Although we went through the most important features, there is still a lot to discover. Probably the best way to learn more is to get your hands dirty with some real-life data. It is also a good idea to go through pandas' excellent documentation, in particular the Cookbook.