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# Intro to data structures

We’ll start with a quick, non-comprehensive overview of the fundamental data structures in pandas to get you started. The fundamental behavior about data types, indexing, axis labeling, and alignment apply across all of the objects. To get started, import NumPy and load pandas into your namespace:

    In [1]: import numpy as np

    In [2]: import pandas as pd

Fundamentally, data alignment is intrinsic. The link between labels and data will not be broken unless done so explicitly by you.

We’ll give a brief intro to the data structures, then consider all of the broad categories of functionality and methods in separate sections.

## Series

`Series` is a one-dimensional labeled array capable of holding any data type (integers, strings, floating point numbers, Python objects, etc.). The axis labels are collectively referred to as the index. The basic method to create a `Series` is to call:

    s = pd.Series(data, index=index)

Here, `data` can be many different things:

- a Python dict

- an ndarray

- a scalar value (like 5)

The passed index is a list of axis labels. The constructor’s behavior depends on data’s type:

From ndarray

If `data` is an ndarray, index must be the same length as data. If no index is passed, one will be created having values `[0, ..., len(data) - 1]`.

    In [3]: s = pd.Series(np.random.randn(5), index=["a", "b", "c", "d", "e"])

    In [4]: s
    Out[4]:
    a    0.469112
    b   -0.282863
    c   -1.509059
    d   -1.135632
    e    1.212112
    dtype: float64

    In [5]: s.index
    Out[5]: Index(['a', 'b', 'c', 'd', 'e'], dtype='str')

    In [6]: pd.Series(np.random.randn(5))
    Out[6]:
    0   -0.173215
    1    0.119209
    2   -1.044236
    3   -0.861849
    4   -2.104569
    dtype: float64

Note

pandas supports non-unique index values. If an operation that does not support duplicate index values is attempted, an exception will be raised at that time.

From dict

`Series` can be instantiated from dicts:

    In [7]: d = {"b": 1, "a": 0, "c": 2}

    In [8]: pd.Series(d)
    Out[8]:
    b    1
    a    0
    c    2
    dtype: int64

If an index is passed, the values in data corresponding to the labels in the index will be pulled out.

    In [9]: d = {"a": 0.0, "b": 1.0, "c": 2.0}

    In [10]: pd.Series(d)
    Out[10]:
    a    0.0
    b    1.0
    c    2.0
    dtype: float64

    In [11]: pd.Series(d, index=["b", "c", "d", "a"])
    Out[11]:
    b    1.0
    c    2.0
    d    NaN
    a    0.0
    dtype: float64

Note

NaN (not a number) is the standard missing data marker used in pandas.

From scalar value

If `data` is a scalar value, the value will be repeated to match the length of index. If the index is not provided, it defaults to `RangeIndex(1)`.

    In [12]: pd.Series(5.0, index=["a", "b", "c", "d", "e"])
    Out[12]:
    a    5.0
    b    5.0
    c    5.0
    d    5.0
    e    5.0
    dtype: float64

### Series is ndarray-like

`Series` acts very similarly to a `numpy.ndarray` and is a valid argument to most NumPy functions. However, operations such as slicing will also slice the index.

    In [13]: s.iloc[0]
    Out[13]: np.float64(0.4691122999071863)

    In [14]: s.iloc[:3]
    Out[14]:
    a    0.469112
    b   -0.282863
    c   -1.509059
    dtype: float64

    In [15]: s[s > s.median()]
    Out[15]:
    a    0.469112
    e    1.212112
    dtype: float64

    In [16]: s.iloc[[4, 3, 1]]
    Out[16]:
    e    1.212112
    d   -1.135632
    b   -0.282863
    dtype: float64

    In [17]: np.exp(s)
    Out[17]:
    a    1.598575
    b    0.753623
    c    0.221118
    d    0.321219
    e    3.360575
    dtype: float64

Note

We will address array-based indexing like `s.iloc[[4, 3, 1]]` in the section on indexing.

Like a NumPy array, a pandas `Series` has a single `dtype`.

    In [18]: s.dtype
    Out[18]: dtype('float64')

This is often a NumPy dtype. However, pandas and 3rd-party libraries extend NumPy’s type system in a few places, in which case the dtype would be an `ExtensionDtype`. Some examples within pandas are Categorical data and Nullable integer data type. See dtypes for more.

If you need the actual array backing a `Series`, use `Series.array`.

    In [19]: s.array
    Out[19]:
    <NumpyExtensionArray>
    [ 0.4691122999071863, -0.2828633443286633, -1.5090585031735124,
     -1.1356323710171934,  1.2121120250208506]
    Length: 5, dtype: float64

Accessing the array can be useful when you need to do some operation without the index (to disable automatic alignment, for example).

`Series.array` will always be an `ExtensionArray`. Briefly, an ExtensionArray is a thin wrapper around one or more concrete arrays like a `numpy.ndarray`. pandas knows how to take an `ExtensionArray` and store it in a `Series` or a column of a `DataFrame`. See dtypes for more.

While `Series` is ndarray-like, if you need an actual ndarray, then use `Series.to_numpy()`.

    In [20]: s.to_numpy()
    Out[20]: array([ 0.4691, -0.2829, -1.5091, -1.1356,  1.2121])

Even if the `Series` is backed by a `ExtensionArray`, `Series.to_numpy()` will return a NumPy ndarray.

### Series is dict-like

A `Series` is also like a fixed-size dict in that you can get and set values by index label:

    In [21]: s["a"]
    Out[21]: np.float64(0.4691122999071863)

    In [22]: s["e"] = 12.0

    In [23]: s
    Out[23]:
    a     0.469112
    b    -0.282863
    c    -1.509059
    d    -1.135632
    e    12.000000
    dtype: float64

    In [24]: "e" in s
    Out[24]: True

    In [25]: "f" in s
    Out[25]: False

If a label is not contained in the index, an exception is raised:

    In [26]: s["f"]
    ---------------------------------------------------------------------------
    KeyError                                  Traceback (most recent call last)
    File ~/work/pandas/pandas/pandas/core/indexes/base.py:3631, in Index.get_loc(self, key)
       3630 try:
    -> 3631     return self._engine.get_loc(casted_key)
       3632 except KeyError as err:

    File ~/work/pandas/pandas/pandas/_libs/index.pyx:172, in pandas._libs.index.IndexEngine.get_loc()

    File ~/work/pandas/pandas/pandas/_libs/index.pyx:201, in pandas._libs.index.IndexEngine.get_loc()

    File pandas/_libs/hashtable_class_helper.pxi:7697, in pandas._libs.hashtable.PyObjectHashTable.get_item()

    File pandas/_libs/hashtable_class_helper.pxi:7705, in pandas._libs.hashtable.PyObjectHashTable.get_item()

    KeyError: 'f'

    The above exception was the direct cause of the following exception:

    KeyError                                  Traceback (most recent call last)
    Cell In[26], line 1
    ----> 1 s["f"]

    File ~/work/pandas/pandas/pandas/core/series.py:1009, in Series.__getitem__(self, key)
       1004     key = unpack_1tuple(key)
       1006 elif key_is_scalar:
       1007     # Note: GH#50617 in 3.0 we changed int key to always be treated as
       1008     #  a label, matching DataFrame behavior.
    -> 1009     return self._get_value(key)
       1011 # Convert generator to list before going through hashable part
       1012 # (We will iterate through the generator there to check for slices)
       1013 if is_iterator(key):

    File ~/work/pandas/pandas/pandas/core/series.py:1096, in Series._get_value(self, label, takeable)
       1093     return self._values[label]
       1095 # Similar to Index.get_value, but we do not fall back to positional
    -> 1096 loc = self.index.get_loc(label)
       1098 if is_integer(loc):
       1099     return self._values[loc]

    File ~/work/pandas/pandas/pandas/core/indexes/base.py:3638, in Index.get_loc(self, key)
       3633     if isinstance(casted_key, slice) or (
       3634         isinstance(casted_key, abc.Iterable)
       3635         and any(isinstance(x, slice) for x in casted_key)
       3636     ):
       3637         raise InvalidIndexError(key) from err
    -> 3638     raise KeyError(key) from err
       3639 except TypeError:
       3640     # If we have a listlike key, _check_indexing_error will raise
       3641     #  InvalidIndexError. Otherwise we fall through and re-raise
       3642     #  the TypeError.
       3643     self._check_indexing_error(key)

    KeyError: 'f'

Using the `Series.get()` method, a missing label will return None or specified default:

    In [27]: s.get("f")

    In [28]: s.get("f", np.nan)
    Out[28]: nan

These labels can also be accessed by attribute.

### Vectorized operations and label alignment with Series

When working with raw NumPy arrays, looping through value-by-value is usually not necessary. The same is true when working with `Series` in pandas. `Series` can also be passed into most NumPy methods expecting an ndarray.

    In [29]: s + s
    Out[29]:
    a     0.938225
    b    -0.565727
    c    -3.018117
    d    -2.271265
    e    24.000000
    dtype: float64

    In [30]: s * 2
    Out[30]:
    a     0.938225
    b    -0.565727
    c    -3.018117
    d    -2.271265
    e    24.000000
    dtype: float64

    In [31]: np.exp(s)
    Out[31]:
    a         1.598575
    b         0.753623
    c         0.221118
    d         0.321219
    e    162754.791419
    dtype: float64

A key difference between `Series` and ndarray is that operations between `Series` automatically align the data based on label. Thus, you can write computations without giving consideration to whether the `Series` involved have the same labels.

    In [32]: s.iloc[1:] + s.iloc[:-1]
    Out[32]:
    a         NaN
    b   -0.565727
    c   -3.018117
    d   -2.271265
    e         NaN
    dtype: float64

The result of an operation between unaligned `Series` will have the union of the indexes involved. If a label is not found in one `Series` or the other, the result will be marked as missing `NaN`. Being able to write code without doing any explicit data alignment grants immense freedom and flexibility in interactive data analysis and research. The integrated data alignment features of the pandas data structures set pandas apart from the majority of related tools for working with labeled data.

Note

In general, we chose to make the default result of operations between differently indexed objects yield the union of the indexes in order to avoid loss of information. Having an index label, though the data is missing, is typically important information as part of a computation. You of course have the option of dropping labels with missing data via the dropna function.

### Name attribute

`Series` also has a `name` attribute:

    In [33]: s = pd.Series(np.random.randn(5), name="something")

    In [34]: s
    Out[34]:
    0   -0.494929
    1    1.071804
    2    0.721555
    3   -0.706771
    4   -1.039575
    Name: something, dtype: float64

    In [35]: s.name
    Out[35]: 'something'

The `Series` `name` can be assigned automatically in many cases, in particular, when selecting a single column from a `DataFrame`, the `name` will be assigned the column label.

You can rename a `Series` with the `pandas.Series.rename()` method.

    In [36]: s2 = s.rename("different")

    In [37]: s2.name
    Out[37]: 'different'

Note that `s` and `s2` refer to different objects.

## DataFrame

`DataFrame` is a 2-dimensional labeled data structure with columns of potentially different types. You can think of it like a spreadsheet or SQL table, or a dict of Series objects. It is generally the most commonly used pandas object. Like Series, DataFrame accepts many different kinds of input:

- Dict of 1D ndarrays, lists, dicts, or `Series`

- 2-D numpy.ndarray

- Structured or record ndarray

- A `Series`

- Another `DataFrame`

Along with the data, you can optionally pass index (row labels) and columns (column labels) arguments. If you pass an index and / or columns, you are guaranteeing the index and / or columns of the resulting DataFrame. Thus, a dict of Series plus a specific index will discard all data not matching up to the passed index.

If axis labels are not passed, they will be constructed from the input data based on common sense rules.

### From dict of Series or dicts

The resulting index will be the union of the indexes of the various Series. If there are any nested dicts, these will first be converted to Series. If no columns are passed, the columns will be the ordered list of dict keys.

    In [38]: d = {
       ....:     "one": pd.Series([1.0, 2.0, 3.0], index=["a", "b", "c"]),
       ....:     "two": pd.Series([1.0, 2.0, 3.0, 4.0], index=["a", "b", "c", "d"]),
       ....: }
       ....:

    In [39]: df = pd.DataFrame(d)

    In [40]: df
    Out[40]:
       one  two
    a  1.0  1.0
    b  2.0  2.0
    c  3.0  3.0
    d  NaN  4.0

    In [41]: pd.DataFrame(d, index=["d", "b", "a"])
    Out[41]:
       one  two
    d  NaN  4.0
    b  2.0  2.0
    a  1.0  1.0

    In [42]: pd.DataFrame(d, index=["d", "b", "a"], columns=["two", "three"])
    Out[42]:
       two three
    d  4.0   NaN
    b  2.0   NaN
    a  1.0   NaN

The row and column labels can be accessed respectively by accessing the index and columns attributes:

Note

When a particular set of columns is passed along with a dict of data, the passed columns override the keys in the dict.

    In [43]: df.index
    Out[43]: Index(['a', 'b', 'c', 'd'], dtype='str')

    In [44]: df.columns
    Out[44]: Index(['one', 'two'], dtype='str')

### From dict of ndarrays / lists

All ndarrays must share the same length. If an index is passed, it must also be the same length as the arrays. If no index is passed, the result will be `range(n)`, where `n` is the array length.

    In [45]: d = {"one": [1.0, 2.0, 3.0, 4.0], "two": [4.0, 3.0, 2.0, 1.0]}

    In [46]: pd.DataFrame(d)
    Out[46]:
       one  two
    0  1.0  4.0
    1  2.0  3.0
    2  3.0  2.0
    3  4.0  1.0

    In [47]: pd.DataFrame(d, index=["a", "b", "c", "d"])
    Out[47]:
       one  two
    a  1.0  4.0
    b  2.0  3.0
    c  3.0  2.0
    d  4.0  1.0

### From structured or record array

This case is handled identically to a dict of arrays.

    In [48]: data = np.zeros((2,), dtype=[("A", "i4"), ("B", "f4"), ("C", "S10")])

    In [49]: data[:] = [(1, 2.0, "Hello"), (2, 3.0, "World")]

    In [50]: pd.DataFrame(data)
    Out[50]:
       A    B         C
    0  1  2.0  b'Hello'
    1  2  3.0  b'World'

    In [51]: pd.DataFrame(data, index=["first", "second"])
    Out[51]:
            A    B         C
    first   1  2.0  b'Hello'
    second  2  3.0  b'World'

    In [52]: pd.DataFrame(data, columns=["C", "A", "B"])
    Out[52]:
              C  A    B
    0  b'Hello'  1  2.0
    1  b'World'  2  3.0

Note

DataFrame is not intended to work exactly like a 2-dimensional NumPy ndarray.

### From a list of dicts

    In [53]: data2 = [{"a": 1, "b": 2}, {"a": 5, "b": 10, "c": 20}]

    In [54]: pd.DataFrame(data2)
    Out[54]:
       a   b     c
    0  1   2   NaN
    1  5  10  20.0

    In [55]: pd.DataFrame(data2, index=["first", "second"])
    Out[55]:
            a   b     c
    first   1   2   NaN
    second  5  10  20.0

    In [56]: pd.DataFrame(data2, columns=["a", "b"])
    Out[56]:
       a   b
    0  1   2
    1  5  10

### From a dict of tuples

You can automatically create a MultiIndexed frame by passing a tuples dictionary.

    In [57]: pd.DataFrame(
       ....:     {
       ....:         ("a", "b"): {("A", "B"): 1, ("A", "C"): 2},
       ....:         ("a", "a"): {("A", "C"): 3, ("A", "B"): 4},
       ....:         ("a", "c"): {("A", "B"): 5, ("A", "C"): 6},
       ....:         ("b", "a"): {("A", "C"): 7, ("A", "B"): 8},
       ....:         ("b", "b"): {("A", "D"): 9, ("A", "B"): 10},
       ....:     }
       ....: )
       ....:
    Out[57]:
           a              b
           b    a    c    a     b
    A B  1.0  4.0  5.0  8.0  10.0
      C  2.0  3.0  6.0  7.0   NaN
      D  NaN  NaN  NaN  NaN   9.0

### From a Series

The result will be a DataFrame with the same index as the input Series, and with one column whose name is the original name of the Series (only if no other column name provided).

    In [58]: ser = pd.Series(range(3), index=list("abc"), name="ser")

    In [59]: pd.DataFrame(ser)
    Out[59]:
       ser
    a    0
    b    1
    c    2

### From a list of namedtuples

The field names of the first `namedtuple` in the list determine the columns of the `DataFrame`. The remaining namedtuples (or tuples) are simply unpacked and their values are fed into the rows of the `DataFrame`. If any of those tuples is shorter than the first `namedtuple` then the later columns in the corresponding row are marked as missing values. If any are longer than the first `namedtuple`, a `ValueError` is raised.

    In [60]: from collections import namedtuple

    In [61]: Point = namedtuple("Point", "x y")

    In [62]: pd.DataFrame([Point(0, 0), Point(0, 3), (2, 3)])
    Out[62]:
       x  y
    0  0  0
    1  0  3
    2  2  3

    In [63]: Point3D = namedtuple("Point3D", "x y z")

    In [64]: pd.DataFrame([Point3D(0, 0, 0), Point3D(0, 3, 5), Point(2, 3)])
    Out[64]:
       x  y    z
    0  0  0  0.0
    1  0  3  5.0
    2  2  3  NaN

### From a list of dataclasses

Data Classes as introduced in PEP557, can be passed into the DataFrame constructor. Passing a list of dataclasses is equivalent to passing a list of dictionaries.

Please be aware, that all values in the list should be dataclasses, mixing types in the list would result in a `TypeError`.

    In [65]: from dataclasses import make_dataclass

    In [66]: Point = make_dataclass("Point", [("x", int), ("y", int)])

    In [67]: pd.DataFrame([Point(0, 0), Point(0, 3), Point(2, 3)])
    Out[67]:
       x  y
    0  0  0
    1  0  3
    2  2  3

Missing data

To construct a DataFrame with missing data, we use `np.nan` to represent missing values. Alternatively, you may pass a `numpy.MaskedArray` as the data argument to the DataFrame constructor, and its masked entries will be considered missing. See Missing data for more.

### Alternate constructors

DataFrame.from_dict

`DataFrame.from_dict()` takes a dict of dicts or a dict of array-like sequences and returns a DataFrame. It operates like the `DataFrame` constructor except for the `orient` parameter which is `'columns'` by default, but which can be set to `'index'` in order to use the dict keys as row labels.

    In [68]: pd.DataFrame.from_dict(dict([("A", [1, 2, 3]), ("B", [4, 5, 6])]))
    Out[68]:
       A  B
    0  1  4
    1  2  5
    2  3  6

If you pass `orient='index'`, the keys will be the row labels. In this case, you can also pass the desired column names:

    In [69]: pd.DataFrame.from_dict(
       ....:     dict([("A", [1, 2, 3]), ("B", [4, 5, 6])]),
       ....:     orient="index",
       ....:     columns=["one", "two", "three"],
       ....: )
       ....:
    Out[69]:
       one  two  three
    A    1    2      3
    B    4    5      6

DataFrame.from_records

`DataFrame.from_records()` takes a list of tuples or an ndarray with structured dtype. It works analogously to the normal `DataFrame` constructor, except that the resulting DataFrame index may be a specific field of the structured dtype.

    In [70]: data
    Out[70]:
    array([(1, 2., b'Hello'), (2, 3., b'World')],
          dtype=[('A', '<i4'), ('B', '<f4'), ('C', 'S10')])

    In [71]: pd.DataFrame.from_records(data, index="C")
    Out[71]:
              A    B
    C
    b'Hello'  1  2.0
    b'World'  2  3.0

### Column selection, addition, deletion

You can treat a `DataFrame` semantically like a dict of like-indexed `Series` objects. Getting, setting, and deleting columns works with the same syntax as the analogous dict operations:

    In [72]: df["one"]
    Out[72]:
    a    1.0
    b    2.0
    c    3.0
    d    NaN
    Name: one, dtype: float64

    In [73]: df["three"] = df["one"] * df["two"]

    In [74]: df["flag"] = df["one"] > 2

    In [75]: df
    Out[75]:
       one  two  three   flag
    a  1.0  1.0    1.0  False
    b  2.0  2.0    4.0  False
    c  3.0  3.0    9.0   True
    d  NaN  4.0    NaN  False

Columns can be deleted or popped like with a dict:

    In [76]: del df["two"]

    In [77]: three = df.pop("three")

    In [78]: df
    Out[78]:
       one   flag
    a  1.0  False
    b  2.0  False
    c  3.0   True
    d  NaN  False

When inserting a scalar value, it will naturally be propagated to fill the column:

    In [79]: df["foo"] = "bar"

    In [80]: df
    Out[80]:
       one   flag  foo
    a  1.0  False  bar
    b  2.0  False  bar
    c  3.0   True  bar
    d  NaN  False  bar

When inserting a `Series` that does not have the same index as the `DataFrame`, it will be conformed to the DataFrame’s index:

    In [81]: df["one_trunc"] = df["one"][:2]

    In [82]: df
    Out[82]:
       one   flag  foo  one_trunc
    a  1.0  False  bar        1.0
    b  2.0  False  bar        2.0
    c  3.0   True  bar        NaN
    d  NaN  False  bar        NaN

You can insert raw ndarrays but their length must match the length of the DataFrame’s index.

By default, columns get inserted at the end. `DataFrame.insert()` inserts at a particular location in the columns:

    In [83]: df.insert(1, "bar", df["one"])

    In [84]: df
    Out[84]:
       one  bar   flag  foo  one_trunc
    a  1.0  1.0  False  bar        1.0
    b  2.0  2.0  False  bar        2.0
    c  3.0  3.0   True  bar        NaN
    d  NaN  NaN  False  bar        NaN

### Assigning new columns in method chains

Inspired by dplyr’s `mutate` verb, DataFrame has an `assign()` method that allows you to easily create new columns that are potentially derived from existing columns.

    In [85]: iris = pd.read_csv("data/iris.data")

    In [86]: iris.head()
    Out[86]:
       SepalLength  SepalWidth  PetalLength  PetalWidth         Name
    0          5.1         3.5          1.4         0.2  Iris-setosa
    1          4.9         3.0          1.4         0.2  Iris-setosa
    2          4.7         3.2          1.3         0.2  Iris-setosa
    3          4.6         3.1          1.5         0.2  Iris-setosa
    4          5.0         3.6          1.4         0.2  Iris-setosa

    In [87]: iris.assign(sepal_ratio=iris["SepalWidth"] / iris["SepalLength"]).head()
    Out[87]:
       SepalLength  SepalWidth  PetalLength  PetalWidth         Name  sepal_ratio
    0          5.1         3.5          1.4         0.2  Iris-setosa     0.686275
    1          4.9         3.0          1.4         0.2  Iris-setosa     0.612245
    2          4.7         3.2          1.3         0.2  Iris-setosa     0.680851
    3          4.6         3.1          1.5         0.2  Iris-setosa     0.673913
    4          5.0         3.6          1.4         0.2  Iris-setosa     0.720000

In the example above, we inserted a precomputed value. We can also pass in a function of one argument to be evaluated on the DataFrame being assigned to.

    In [88]: iris.assign(sepal_ratio=lambda x: (x["SepalWidth"] / x["SepalLength"])).head()
    Out[88]:
       SepalLength  SepalWidth  PetalLength  PetalWidth         Name  sepal_ratio
    0          5.1         3.5          1.4         0.2  Iris-setosa     0.686275
    1          4.9         3.0          1.4         0.2  Iris-setosa     0.612245
    2          4.7         3.2          1.3         0.2  Iris-setosa     0.680851
    3          4.6         3.1          1.5         0.2  Iris-setosa     0.673913
    4          5.0         3.6          1.4         0.2  Iris-setosa     0.720000

or, using `pandas.col()`:

    In [89]: iris.assign(sepal_ratio=pd.col("SepalWidth") / pd.col("SepalLength")).head()
    Out[89]:
       SepalLength  SepalWidth  PetalLength  PetalWidth         Name  sepal_ratio
    0          5.1         3.5          1.4         0.2  Iris-setosa     0.686275
    1          4.9         3.0          1.4         0.2  Iris-setosa     0.612245
    2          4.7         3.2          1.3         0.2  Iris-setosa     0.680851
    3          4.6         3.1          1.5         0.2  Iris-setosa     0.673913
    4          5.0         3.6          1.4         0.2  Iris-setosa     0.720000

`assign()` always returns a copy of the data, leaving the original DataFrame untouched.

Passing a callable, as opposed to an actual value to be inserted, is useful when you don’t have a reference to the DataFrame at hand. This is common when using `assign()` in a chain of operations. For example, we can limit the DataFrame to just those observations with a Sepal Length greater than 5, calculate the ratio, and plot:

    In [90]: (
       ....:     iris.query("SepalLength > 5")
       ....:     .assign(
       ....:         SepalRatio=lambda x: x.SepalWidth / x.SepalLength,
       ....:         PetalRatio=lambda x: x.PetalWidth / x.PetalLength,
       ....:     )
       ....:     .plot(kind="scatter", x="SepalRatio", y="PetalRatio")
       ....: )
       ....:
    Out[90]: <Axes: xlabel='SepalRatio', ylabel='PetalRatio'>

Since a function is passed in, the function is computed on the DataFrame being assigned to. Importantly, this is the DataFrame that’s been filtered to those rows with sepal length greater than 5. The filtering happens first, and then the ratio calculations. This is an example where we didn’t have a reference to the filtered DataFrame available.

The function signature for `assign()` is simply `**kwargs`. The keys are the column names for the new fields, and the values are either a value to be inserted (for example, a `Series` or NumPy array), or a function of one argument to be called on the `DataFrame`. A copy of the original `DataFrame` is returned, with the new values inserted.

The order of `**kwargs` is preserved. This allows for dependent assignment, where an expression later in `**kwargs` can refer to a column created earlier in the same `assign()`.

    In [91]: dfa = pd.DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]})

    In [92]: dfa.assign(C=lambda x: x["A"] + x["B"], D=lambda x: x["A"] + x["C"])
    Out[92]:
       A  B  C   D
    0  1  4  5   6
    1  2  5  7   9
    2  3  6  9  12

In the second expression, `x['C']` will refer to the newly created column, that’s equal to `dfa['A'] + dfa['B']`.

### Indexing / selection

The basics of indexing are as follows:

Operation | Syntax | Result
--- | --- | ---
Select column | `df[col]` | Series
Select row by label | `df.loc[label]` | Series
Select row by integer location | `df.iloc[loc]` | Series
Slice rows | `df[5:10]` | DataFrame
Select rows by boolean vector | `df[bool_vec]` | DataFrame

Row selection, for example, returns a `Series` whose index is the columns of the `DataFrame`:

    In [93]: df.loc["b"]
    Out[93]:
    one            2.0
    bar            2.0
    flag         False
    foo            bar
    one_trunc      2.0
    Name: b, dtype: object

    In [94]: df.iloc[2]
    Out[94]:
    one           3.0
    bar           3.0
    flag         True
    foo           bar
    one_trunc     NaN
    Name: c, dtype: object

For a more exhaustive treatment of sophisticated label-based indexing and slicing, see the section on indexing. We will address the fundamentals of reindexing / conforming to new sets of labels in the section on reindexing.

### Data alignment and arithmetic

Data alignment between `DataFrame` objects automatically align on both the columns and the index (row labels). Again, the resulting object will have the union of the column and row labels.

    In [95]: df = pd.DataFrame(np.random.randn(10, 4), columns=["A", "B", "C", "D"])

    In [96]: df2 = pd.DataFrame(np.random.randn(7, 3), columns=["A", "B", "C"])

    In [97]: df + df2
    Out[97]:
              A         B         C   D
    0  0.045691 -0.014138  1.380871 NaN
    1 -0.955398 -1.501007  0.037181 NaN
    2 -0.662690  1.534833 -0.859691 NaN
    3 -2.452949  1.237274 -0.133712 NaN
    4  1.414490  1.951676 -2.320422 NaN
    5 -0.494922 -1.649727 -1.084601 NaN
    6 -1.047551 -0.748572 -0.805479 NaN
    7       NaN       NaN       NaN NaN
    8       NaN       NaN       NaN NaN
    9       NaN       NaN       NaN NaN

When doing an operation between `DataFrame` and `Series`, the default behavior is to align the `Series` index on the `DataFrame` columns, thus broadcasting row-wise. For example:

    In [98]: df - df.iloc[0]
    Out[98]:
              A         B         C         D
    0  0.000000  0.000000  0.000000  0.000000
    1 -1.359261 -0.248717 -0.453372 -1.754659
    2  0.253128  0.829678  0.010026 -1.991234
    3 -1.311128  0.054325 -1.724913 -1.620544
    4  0.573025  1.500742 -0.676070  1.367331
    5 -1.741248  0.781993 -1.241620 -2.053136
    6 -1.240774 -0.869551 -0.153282  0.000430
    7 -0.743894  0.411013 -0.929563 -0.282386
    8 -1.194921  1.320690  0.238224 -1.482644
    9  2.293786  1.856228  0.773289 -1.446531

For explicit control over the matching and broadcasting behavior, see the section on flexible binary operations.

Arithmetic operations with scalars operate element-wise:

    In [99]: df * 5 + 2
    Out[99]:
               A         B         C          D
    0   3.359299 -0.124862  4.835102   3.381160
    1  -3.437003 -1.368449  2.568242  -5.392133
    2   4.624938  4.023526  4.885230  -6.575010
    3  -3.196342  0.146766 -3.789461  -4.721559
    4   6.224426  7.378849  1.454750  10.217815
    5  -5.346940  3.785103 -1.373001  -6.884519
    6  -2.844569 -4.472618  4.068691   3.383309
    7  -0.360173  1.930201  0.187285   1.969232
    8  -2.615303  6.478587  6.026220  -4.032059
    9  14.828230  9.156280  8.701544  -3.851494

    In [100]: 1 / df
    Out[100]:
              A          B         C           D
    0  3.678365  -2.353094  1.763605    3.620145
    1 -0.919624  -1.484363  8.799067   -0.676395
    2  1.904807   2.470934  1.732964   -0.583090
    3 -0.962215  -2.697986 -0.863638   -0.743875
    4  1.183593   0.929567 -9.170108    0.608434
    5 -0.680555   2.800959 -1.482360   -0.562777
    6 -1.032084  -0.772485  2.416988    3.614523
    7 -2.118489 -71.634509 -2.758294 -162.507295
    8 -1.083352   1.116424  1.241860   -0.828904
    9  0.389765   0.698687  0.746097   -0.854483

    In [101]: df ** 4
    Out[101]:
               A             B         C             D
    0   0.005462  3.261689e-02  0.103370  5.822320e-03
    1   1.398165  2.059869e-01  0.000167  4.777482e+00
    2   0.075962  2.682596e-02  0.110877  8.650845e+00
    3   1.166571  1.887302e-02  1.797515  3.265879e+00
    4   0.509555  1.339298e+00  0.000141  7.297019e+00
    5   4.661717  1.624699e-02  0.207103  9.969092e+00
    6   0.881334  2.808277e+00  0.029302  5.858632e-03
    7   0.049647  3.797614e-08  0.017276  1.433866e-09
    8   0.725974  6.437005e-01  0.420446  2.118275e+00
    9  43.329821  4.196326e+00  3.227153  1.875802e+00

Boolean operators operate element-wise as well:

    In [102]: df1 = pd.DataFrame({"a": [1, 0, 1], "b": [0, 1, 1]}, dtype=bool)

    In [103]: df2 = pd.DataFrame({"a": [0, 1, 1], "b": [1, 1, 0]}, dtype=bool)

    In [104]: df1 & df2
    Out[104]:
           a      b
    0  False  False
    1  False   True
    2   True  False

    In [105]: df1 | df2
    Out[105]:
          a     b
    0  True  True
    1  True  True
    2  True  True

    In [106]: df1 ^ df2
    Out[106]:
           a      b
    0   True   True
    1   True  False
    2  False   True

    In [107]: -df1
    Out[107]:
           a      b
    0  False   True
    1   True  False
    2  False  False

### Transposing

To transpose, access the `T` attribute or `DataFrame.transpose()`, similar to an ndarray:

    # only show the first 5 rows
    In [108]: df[:5].T
    Out[108]:
              0         1         2         3         4
    A  0.271860 -1.087401  0.524988 -1.039268  0.844885
    B -0.424972 -0.673690  0.404705 -0.370647  1.075770
    C  0.567020  0.113648  0.577046 -1.157892 -0.109050
    D  0.276232 -1.478427 -1.715002 -1.344312  1.643563

### DataFrame interoperability with NumPy functions

Most NumPy functions can be called directly on `Series` and `DataFrame`.

    In [109]: np.exp(df)
    Out[109]:
               A         B         C         D
    0   1.312403  0.653788  1.763006  1.318154
    1   0.337092  0.509824  1.120358  0.227996
    2   1.690438  1.498861  1.780770  0.179963
    3   0.353713  0.690288  0.314148  0.260719
    4   2.327710  2.932249  0.896686  5.173571
    5   0.230066  1.429065  0.509360  0.169161
    6   0.379495  0.274028  1.512461  1.318720
    7   0.623732  0.986137  0.695904  0.993865
    8   0.397301  2.449092  2.237242  0.299269
    9  13.009059  4.183951  3.820223  0.310274

    In [110]: np.asarray(df)
    Out[110]:
    array([[ 0.2719, -0.425 ,  0.567 ,  0.2762],
           [-1.0874, -0.6737,  0.1136, -1.4784],
           [ 0.525 ,  0.4047,  0.577 , -1.715 ],
           [-1.0393, -0.3706, -1.1579, -1.3443],
           [ 0.8449,  1.0758, -0.109 ,  1.6436],
           [-1.4694,  0.357 , -0.6746, -1.7769],
           [-0.9689, -1.2945,  0.4137,  0.2767],
           [-0.472 , -0.014 , -0.3625, -0.0062],
           [-0.9231,  0.8957,  0.8052, -1.2064],
           [ 2.5656,  1.4313,  1.3403, -1.1703]])

`DataFrame` is not intended to be a drop-in replacement for ndarray as its indexing semantics and data model are quite different in places from an n-dimensional array.

`Series` implements `__array_ufunc__`, which allows it to work with NumPy’s universal functions.

The ufunc is applied to the underlying array in a `Series`.

    In [111]: ser = pd.Series([1, 2, 3, 4])

    In [112]: np.exp(ser)
    Out[112]:
    0     2.718282
    1     7.389056
    2    20.085537
    3    54.598150
    dtype: float64

When multiple `Series` are passed to a ufunc, they are aligned before performing the operation.

Like other parts of the library, pandas will automatically align labeled inputs as part of a ufunc with multiple inputs. For example, using `numpy.remainder()` on two `Series` with differently ordered labels will align before the operation.

    In [113]: ser1 = pd.Series([1, 2, 3], index=["a", "b", "c"])

    In [114]: ser2 = pd.Series([1, 3, 5], index=["b", "a", "c"])

    In [115]: ser1
    Out[115]:
    a    1
    b    2
    c    3
    dtype: int64

    In [116]: ser2
    Out[116]:
    b    1
    a    3
    c    5
    dtype: int64

    In [117]: np.remainder(ser1, ser2)
    Out[117]:
    a    1
    b    0
    c    3
    dtype: int64

As usual, the union of the two indices is taken, and non-overlapping values are filled with missing values.

    In [118]: ser3 = pd.Series([2, 4, 6], index=["b", "c", "d"])

    In [119]: ser3
    Out[119]:
    b    2
    c    4
    d    6
    dtype: int64

    In [120]: np.remainder(ser1, ser3)
    Out[120]:
    a    NaN
    b    0.0
    c    3.0
    d    NaN
    dtype: float64

When a binary ufunc is applied to a `Series` and `Index`, the `Series` implementation takes precedence and a `Series` is returned.

    In [121]: ser = pd.Series([1, 2, 3])

    In [122]: idx = pd.Index([4, 5, 6])

    In [123]: np.maximum(ser, idx)
    Out[123]:
    0    4
    1    5
    2    6
    dtype: int64

NumPy ufuncs are safe to apply to `Series` backed by non-ndarray arrays, for example `arrays.SparseArray` (see Sparse calculation). If possible, the ufunc is applied without converting the underlying data to an ndarray.

### Console display

A very large `DataFrame` will be truncated to display them in the console. You can also get a summary using `info()`. (The baseball dataset is from the plyr R package):

    In [124]: baseball = pd.read_csv("data/baseball.csv")

    In [125]: print(baseball)
           id     player  year  stint team  lg  ...    so  ibb  hbp   sh   sf  gidp
    0   88641  womacto01  2006      2  CHN  NL  ...   4.0  0.0  0.0  3.0  0.0   0.0
    1   88643  schilcu01  2006      1  BOS  AL  ...   1.0  0.0  0.0  0.0  0.0   0.0
    ..    ...        ...   ...    ...  ...  ..  ...   ...  ...  ...  ...  ...   ...
    98  89533   aloumo01  2007      1  NYN  NL  ...  30.0  5.0  2.0  0.0  3.0  13.0
    99  89534  alomasa02  2007      1  NYN  NL  ...   3.0  0.0  0.0  0.0  0.0   0.0

    [100 rows x 23 columns]

    In [126]: baseball.info()
    <class 'pandas.DataFrame'>
    RangeIndex: 100 entries, 0 to 99
    Data columns (total 23 columns):
     #   Column  Non-Null Count  Dtype
    ---  ------  --------------  -----
     0   id      100 non-null    int64
     1   player  100 non-null    str
     2   year    100 non-null    int64
     3   stint   100 non-null    int64
     4   team    100 non-null    str
     5   lg      100 non-null    str
     6   g       100 non-null    int64
     7   ab      100 non-null    int64
     8   r       100 non-null    int64
     9   h       100 non-null    int64
     10  X2b     100 non-null    int64
     11  X3b     100 non-null    int64
     12  hr      100 non-null    int64
     13  rbi     100 non-null    float64
     14  sb      100 non-null    float64
     15  cs      100 non-null    float64
     16  bb      100 non-null    int64
     17  so      100 non-null    float64
     18  ibb     100 non-null    float64
     19  hbp     100 non-null    float64
     20  sh      100 non-null    float64
     21  sf      100 non-null    float64
     22  gidp    100 non-null    float64
    dtypes: float64(9), int64(11), str(3)
    memory usage: 19.5 KB

However, using `DataFrame.to_string()` will return a string representation of the `DataFrame` in tabular form, though it won’t always fit the console width:

    In [127]: print(baseball.iloc[-20:, :12].to_string())
           id     player  year  stint team  lg    g   ab   r    h  X2b  X3b
    80  89474  finlest01  2007      1  COL  NL   43   94   9   17    3    0
    81  89480  embreal01  2007      1  OAK  AL    4    0   0    0    0    0
    82  89481  edmonji01  2007      1  SLN  NL  117  365  39   92   15    2
    83  89482  easleda01  2007      1  NYN  NL   76  193  24   54    6    0
    84  89489  delgaca01  2007      1  NYN  NL  139  538  71  139   30    0
    85  89493  cormirh01  2007      1  CIN  NL    6    0   0    0    0    0
    86  89494  coninje01  2007      2  NYN  NL   21   41   2    8    2    0
    87  89495  coninje01  2007      1  CIN  NL   80  215  23   57   11    1
    88  89497  clemero02  2007      1  NYA  AL    2    2   0    1    0    0
    89  89498  claytro01  2007      2  BOS  AL    8    6   1    0    0    0
    90  89499  claytro01  2007      1  TOR  AL   69  189  23   48   14    0
    91  89501  cirilje01  2007      2  ARI  NL   28   40   6    8    4    0
    92  89502  cirilje01  2007      1  MIN  AL   50  153  18   40    9    2
    93  89521  bondsba01  2007      1  SFN  NL  126  340  75   94   14    0
    94  89523  biggicr01  2007      1  HOU  NL  141  517  68  130   31    3
    95  89525  benitar01  2007      2  FLO  NL   34    0   0    0    0    0
    96  89526  benitar01  2007      1  SFN  NL   19    0   0    0    0    0
    97  89530  ausmubr01  2007      1  HOU  NL  117  349  38   82   16    3
    98  89533   aloumo01  2007      1  NYN  NL   87  328  51  112   19    1
    99  89534  alomasa02  2007      1  NYN  NL    8   22   1    3    1    0

Wide DataFrames will be printed across multiple rows by default:

    In [128]: pd.DataFrame(np.random.randn(3, 12))
    Out[128]:
             0         1         2   ...        9         10        11
    0 -1.226825  0.769804 -1.281247  ... -1.110336 -0.619976  0.149748
    1 -0.732339  0.687738  0.176444  ...  1.462696 -1.743161 -0.826591
    2 -0.345352  1.314232  0.690579  ...  0.896171 -0.487602 -0.082240

    [3 rows x 12 columns]

You can change how much to print on a single row by setting the `display.width` option:

    In [129]: pd.set_option("display.width", 40)  # default is 80

    In [130]: pd.DataFrame(np.random.randn(3, 12))
    Out[130]:
             0         1         2   ...        9         10        11
    0 -2.182937  0.380396  0.084844  ... -0.023688  2.410179  1.450520
    1  0.206053 -0.251905 -2.213588  ... -0.025747 -0.988387  0.094055
    2  1.262731  1.289997  0.082423  ... -0.281461  0.030711  0.109121

    [3 rows x 12 columns]

You can adjust the max width of the individual columns by setting `display.max_colwidth`

    In [131]: datafile = {
       .....:     "filename": ["filename_01", "filename_02"],
       .....:     "path": [
       .....:         "media/user_name/storage/folder_01/filename_01",
       .....:         "media/user_name/storage/folder_02/filename_02",
       .....:     ],
       .....: }
       .....:

    In [132]: pd.set_option("display.max_colwidth", 30)

    In [133]: pd.DataFrame(datafile)
    Out[133]:
          filename                           path
    0  filename_01  media/user_name/storage/fo...
    1  filename_02  media/user_name/storage/fo...

    In [134]: pd.set_option("display.max_colwidth", 100)

    In [135]: pd.DataFrame(datafile)
    Out[135]:
          filename                                           path
    0  filename_01  media/user_name/storage/folder_01/filename_01
    1  filename_02  media/user_name/storage/folder_02/filename_02

You can also disable this feature via the `expand_frame_repr` option. This will print the table in one block.

### DataFrame column attribute access and IPython completion

If a `DataFrame` column label is a valid Python variable name, the column can be accessed like an attribute:

    In [136]: df = pd.DataFrame({"foo1": np.random.randn(5), "foo2": np.random.randn(5)})

    In [137]: df
    Out[137]:
           foo1      foo2
    0  1.126203  0.781836
    1 -0.977349 -1.071357
    2  1.474071  0.441153
    3 -0.064034  2.353925
    4 -1.282782  0.583787

    In [138]: df.foo1
    Out[138]:
    0    1.126203
    1   -0.977349
    2    1.474071
    3   -0.064034
    4   -1.282782
    Name: foo1, dtype: float64

The columns are also connected to the IPython completion mechanism so they can be tab-completed:

    In [5]: df.foo<TAB>  # noqa: E225, E999
    df.foo1  df.foo2