Introduction into Pandas
The pandas we are writing about in this chapter have nothing to do with the cute panda bears, and they are neither what our visitors are expecting in a Python tutorial. Pandas is a Python module, which is rounding up the capabilities of Numpy, Scipy and Matplotlab. The word pandas is an acronym which is derived from "Python and data analysis" and "panel data".
There is often some confusion about whether Pandas is an alternative to Numpy, SciPy and Matplotlib. The truth is that it is built on top of Numpy. This means that Numpy is required by pandas. Scipy and Matplotlib on the other hand are not required by pandas but they are extremely useful. That's why the Pandas project lists them as "optional dependency".
Pandas is a software library written for the Python programming language. It is used for data manipulation and analysis. It provides special data structures and operations for the manipulation of numerical tables and time series. Pandas is free software released under the three-clause BSD license.
Data Structures
We will start with the following two important data structures of Pandas:
- Series and
- DataFrame
Series
A Series is a one-dimensional labelled array-like object. It is capable of holding any data type, e.g. integers, floats, strings, Python objects, and so on. It can be seen as a data structure with two arrays: one functioning as the index, i.e. the labels, and the other one contains the actual data.
We define a simple Series object in the following example by instantiating a Pandas Series object with a list. We will later see that we can use other data objects for example Numpy arrays and dictionaries as well to instantiate a Series object.
import pandas as pd S = pd.Series([11, 28, 72, 3, 5, 8]) SThe above code returned the following:
0 11 1 28 2 72 3 3 4 5 5 8 dtype: int64
We haven't defined an index in our example, but we see two columns in our output: The right column contains our data, whereas the left column contains the index. Pandas created a default index starting with 0 going to 5, which is the length of the data minus 1.
We can directly access the index and the values of our Series S:
print(S.index) print(S.values)
RangeIndex(start=0, stop=6, step=1) [11 28 72 3 5 8]
If we compare this to creating an array in numpy, there are still lots of similarities:
import numpy as np X = np.array([11, 28, 72, 3, 5, 8]) print(X) print(S.values) # both are the same type: print(type(S.values), type(X))
[11 28 72 3 5 8] [11 28 72 3 5 8] <class 'numpy.ndarray'> <class 'numpy.ndarray'>
So far our Series have not been very different to ndarrays of Numpy. This changes, as soon as we start defining Series objects with individual indices:
fruits = ['apples', 'oranges', 'cherries', 'pears'] quantities = [20, 33, 52, 10] S = pd.Series(quantities, index=fruits) SThe previous Python code returned the following output:
apples 20 oranges 33 cherries 52 pears 10 dtype: int64
A big advantage to NumPy arrays is obvious from the previous example: We can use arbitrary indices.
If we add two series with the same indices, we get a new series with the same index and the correponding values will be added:
fruits = ['apples', 'oranges', 'cherries', 'pears'] S = pd.Series([20, 33, 52, 10], index=fruits) S2 = pd.Series([17, 13, 31, 32], index=fruits) print(S + S2) print("sum of S: ", sum(S))
apples 37 oranges 46 cherries 83 pears 42 dtype: int64 sum of S: 115
The indices do not have to be the same for the Series addition. The index will be the "union" of both indices. If an index doesn't occur in both Series, the value for this Series will be NaN:
fruits = ['peaches', 'oranges', 'cherries', 'pears'] fruits2 = ['raspberries', 'oranges', 'cherries', 'pears'] S = pd.Series([20, 33, 52, 10], index=fruits) S2 = pd.Series([17, 13, 31, 32], index=fruits2) print(S + S2)
cherries 83.0 oranges 46.0 peaches NaN pears 42.0 raspberries NaN dtype: float64
fruits = ['apples', 'oranges', 'cherries', 'pears'] fruits_ro = ["mere", "portocale", "cireșe", "pere"] S = pd.Series([20, 33, 52, 10], index=fruits) S2 = pd.Series([17, 13, 31, 32], index=fruits_ro) print(S+S2)
apples NaN cherries NaN cireșe NaN mere NaN oranges NaN pears NaN pere NaN portocale NaN dtype: float64
It's possible to access single values of a Series or more than one value by a list of indices:
print(S['apples'])
20
print(S[['apples', 'oranges', 'cherries']])
apples 20 oranges 33 cherries 52 dtype: int64
Similar to Numpy we can use scalar operations or mathematical functions on a series:
import numpy as np print((S + 3) * 4) print("======================") print(np.sin(S))
apples 92 oranges 144 cherries 220 pears 52 dtype: int64 ====================== apples 0.912945 oranges 0.999912 cherries 0.986628 pears -0.544021 dtype: float64
pandas.Series.apply
Series.apply(func, convert_dtype=True, args=(), **kwds)
The function "func" will be applied to the Series and it returns either a Series or a DataFrame, depending on "func".
| Parameter | Meaning |
|---|---|
| func | a function, which can be a NumPy function that will be applied to the entire Series or a Python function that will be applied to every single value of the series |
| convert_dtype | A boolean value. If it is set to True (default), apply will try to find better dtype for elementwise function results. If False, leave as dtype=object |
| args | Positional arguments which will be passed to the function "func" additionally to the values from the series. |
| **kwds | Additional keyword arguments will be passed as keywords to the function |
Example:
S.apply(np.sin)We received the following result:
apples 0.912945 oranges 0.999912 cherries 0.986628 pears -0.544021 dtype: float64
We can also use Python lambda functions. Let's assume, we have the following task. The test the amount of fruit for every kind. It there are less than 50 available, we will augment the stock by 10:
S.apply(lambda x: x if x > 50 else x+10 )We received the following output:
apples 30 oranges 43 cherries 52 pears 20 dtype: int64
Filtering with a boolean array:
S[S>30]The Python code above returned the following:
oranges 33 cherries 52 dtype: int64
A series can be seen as an ordered Python dictionary with a fixed length.
"apples" in SThe above code returned the following result:
True
We can even pass a dictionary to a Series object, when we create it. We get a Series with the dict's keys as the indices. The indices will be sorted.
cities = {"London": 8615246, "Berlin": 3562166, "Madrid": 3165235, "Rome": 2874038, "Paris": 2273305, "Vienna": 1805681, "Bucharest":1803425, "Hamburg": 1760433, "Budapest": 1754000, "Warsaw": 1740119, "Barcelona":1602386, "Munich": 1493900, "Milan": 1350680} city_series = pd.Series(cities) print(city_series)
Barcelona 1602386 Berlin 3562166 Bucharest 1803425 Budapest 1754000 Hamburg 1760433 London 8615246 Madrid 3165235 Milan 1350680 Munich 1493900 Paris 2273305 Rome 2874038 Vienna 1805681 Warsaw 1740119 dtype: int64
NaN - Missing Data
One problem in dealing with data analysis tasks consists in missing data. Pandas makes it as easy as possible to work with missing data.
If we look once more at our previous example, we can see that the index of our series is the same as the keys of the dictionary we used to create the cities_series. Now, we want to use an index which is not overlapping with the dictionary keys. We have already seen that we can pass a list or a tuple to the keyword argument 'index' to define the index. In our next example, the list (or tuple) passed to the keyword parameter 'index' will not be equal to the keys. This means that some cities from the dictionary will be missing and two cities ("Zurich" and "Stuttgart") don't occur in the dictionary.
my_cities = ["London", "Paris", "Zurich", "Berlin", "Stuttgart", "Hamburg"] my_city_series = pd.Series(cities, index=my_cities) my_city_seriesThis gets us the following result:
London 8615246.0 Paris 2273305.0 Zurich NaN Berlin 3562166.0 Stuttgart NaN Hamburg 1760433.0 dtype: float64
Due to the Nan values the population values for the other cities are turned into floats. There is no missing data in the following examples, so the values are int:
my_cities = ["London", "Paris", "Berlin", "Hamburg"] my_city_series = pd.Series(cities, index=my_cities) my_city_seriesWe received the following output:
London 8615246 Paris 2273305 Berlin 3562166 Hamburg 1760433 dtype: int64
The Methods isnull() and notnull()
We can see, that the cities, which are not included in the dictionary, get the value NaN assigned. NaN stands for "not a number". It can also be seen as meaning "missing" in our example.
We can check for missing values with the methods isnull and notnull:
my_cities = ["London", "Paris", "Zurich", "Berlin", "Stuttgart", "Hamburg"] my_city_series = pd.Series(cities, index=my_cities) print(my_city_series.isnull())
London False Paris False Zurich True Berlin False Stuttgart True Hamburg False dtype: bool
print(my_city_series.notnull())
London True Paris True Zurich False Berlin True Stuttgart False Hamburg True dtype: bool
Connection between NaN and None
We get also a NaN, if a value in the dictionary has a None:
d = {"a":23, "b":45, "c":None, "d":0} S = pd.Series(d) print(S)
a 23.0 b 45.0 c NaN d 0.0 dtype: float64
pd.isnull(S)After having executed the Python code above we received the following result:
a False b False c True d False dtype: bool
pd.notnull(S)This gets us the following:
a True b True c False d True dtype: bool
Filtering out Missing Data
It's possible to filter out missing data with the Series method dropna. It returns a Series which consists only of non-null data:
print(my_city_series.dropna())
London 8615246.0 Paris 2273305.0 Berlin 3562166.0 Hamburg 1760433.0 dtype: float64
Filling in Missing Data
In many cases you don't want to filter out missing data, but you want to fill in appropriate data for the empty gaps. A suitable method in many situations will be fillna:
print(my_city_series.fillna(0))
London 8615246.0 Paris 2273305.0 Zurich 0.0 Berlin 3562166.0 Stuttgart 0.0 Hamburg 1760433.0 dtype: float64
Okay, that's not what we call "fill in appropriate data for the empty gaps". If we call fillna with a dict, we can provide the appropriate data, i.e. the population of Zurich and Stuttgart:
missing_cities = {"Stuttgart":597939, "Zurich":378884} my_city_series.fillna(missing_cities)We received the following output:
London 8615246.0 Paris 2273305.0 Zurich 378884.0 Berlin 3562166.0 Stuttgart 597939.0 Hamburg 1760433.0 dtype: float64
We still have the problem that integer values - which means values which should be integers like number of people - are converted to float as soon as we have NaN values. We can solve this problem now with the method 'fillna':
cities = {"London": 8615246, "Berlin": 3562166, "Madrid": 3165235, "Rome": 2874038, "Paris": 2273305, "Vienna": 1805681, "Bucharest":1803425, "Hamburg": 1760433, "Budapest": 1754000, "Warsaw": 1740119, "Barcelona":1602386, "Munich": 1493900, "Milan": 1350680} my_cities = ["London", "Paris", "Zurich", "Berlin", "Stuttgart", "Hamburg"] my_city_series = pd.Series(cities, index=my_cities) my_city_series = my_city_series.fillna(0).astype(int) print(my_city_series)
London 8615246 Paris 2273305 Zurich 0 Berlin 3562166 Stuttgart 0 Hamburg 1760433 dtype: int64
Selecting Columns from a DataFrame
d = {"a":[2, 4, 12], "b":[12, 3, 9], "c":[1, 1, 1], "d":[0, -2, 7]} d2 = {"a":[5, 7, 15], "b":[1, -1, 9], "c":[-1,1, -1], "d":[0, 2, 3]} df1 = pd.DataFrame(d) df2 = pd.DataFrame(d2) print(df1) df2 = df1[df1.columns[1:3]] print(df2) # alternatively: df3 = df1.iloc[:, 1:3] print(df3)
a b c d
0 2 12 1 0
1 4 3 1 -2
2 12 9 1 7
b c
0 12 1
1 3 1
2 9 1
b c
0 12 1
1 3 1
2 9 1