简化, 并不是逐句翻译, 只关注重点部分, 部分内容加入了自己的理解.

索引和数据选取.

The axis labeling information in pandas objects serves many purposes:

轴标签信息在pandas对象中有多个用途:

In this section, we will focus on the final point: namely, how to slice, dice, and generally get and set subsets of pandas objects. The primary focus will be on Series and DataFrame as they have received more development attention in this area.

在这部分中, 将着重讨论最后一点, 如何对pandas的对象进行切片获取其中的部分数据(即读写操作). 这是pandas的开发优先关注的.

Note 注意

The Python and NumPy indexing operators [] and attribute operator . provide quick and easy access to pandas data structures across a wide range of use cases. This makes interactive work intuitive, as there’s little new to learn if you already know how to deal with Python dictionaries and NumPy arrays. However, since the type of the data to be accessed isn’t known in advance, directly using standard operators has some optimization limits. For production code, we recommended that you take advantage of the optimized pandas data access methods exposed in this chapter.

在python和numpy中, 索引操作符[]和属性操作符逗号, 提供了非常方便的方式访问pandas中的数据在大多数情况下. 如果你python的字典和numpy的数据已经非常熟悉, 这种交互是非常直观明了的.然而, 直接使用标准操作符有一些限制, 这种操作并不是已知最高级的. 对于生产力的代码, 我们推荐你利用本章中提及的方式去访问pandas的数据.

Warning 警告

Whether a copy or a reference is returned for a setting operation, may depend on the context. This is sometimes called chained assignment and should be avoided. See Returning a View versus Copy.

注意链式操作引发的问题.

See the MultiIndex / Advanced Indexing for MultiIndex and more advanced indexing documentation.

查看多索引和高级索引.

See the cookbook for some advanced strategies.

cookbook还有更多的高级应用案例.

索引的不同选择

Object selection has had a number of user-requested additions in order to support more explicit location based indexing. pandas now supports three types of multi-axis indexing.

(数据)对象的获取是用户的高频操作, 现在pandas提供三种不同的多轴索引的支持.

.loc is primarily label based, but may also be used with a boolean array. .loc will raise KeyError when the items are not found. Allowed inputs are:

.loc是基于标签的方式, 通常使用布尔数组. 当用户传入不存在的索引时, 会触发键值错误. 以下是允许输入的索引:

A single label, e.g. 5 or 'a' (Note that 5 is interpreted as a label of the index. This use is not an integer position along the index.).

单个标签, 如数字或者字母, 注意数字在loc中将被当作标签来使用(而不是表示其数字的特性, 指示行的位置, 可以视作文本型的数字).

In [55]: s = pd.Series(list('abcde'), index=[0, 3, 2, 5, 4])

In [56]: s.loc[3:5], 注意这里, 只是表示标签3-标签5之间的数据, 而不是数字意义上3 - 4 - 5 Out[56]: 3 b 2 c 5 d dtype: object

A list or array of labels ['a', 'b', 'c'].

一列数组的标签

A slice object with labels 'a':'f' (Note that contrary to usual Python slices, both the start and the stop are included, when present in the index! See Slicing with labels and Endpoints are inclusive.)

切片式的标签, 注意和python的切片相反, pandas是包含终点部分的数据,.(python是不包含的)

A boolean array (any NA values will be treated as False).

一个布尔数组

A callable function with one argument (the calling Series or DataFrame) and that returns valid output for indexing (one of the above).

一个函数(这个函数会返回上述的索引值内容)

See more at Selection by Label.

更多内容见使用标签选择

.iloc is primarily integer position based (from 0 to length-1 of the axis), but may also be used with a boolean array. .iloc will raise IndexError if a requested indexer is out-of-bounds, except slice indexers which allow out-of-bounds indexing. (this conforms with Python/NumPy slice semantics). Allowed inputs are:

.iloc是基于数字(从 整数0 到 数据长度-1 范围内)的方式. 但是也可以使用布尔型数组.

See more at Selection by Position, Advanced Indexing and Advanced Hierarchical.

.loc, .iloc, and also [] indexing can accept a callable as indexer. See more at Selection By Callable.

上述的三种方式均接受一个函数作为索引.

Getting values from an object with multi-axes selection uses the following notation (using .loc as an example, but the following applies to .iloc as well). Any of the axes accessors may be the null slice :. Axes left out of the specification are assumed to be :, e.g. p.loc['a'] is equivalent to p.loc['a', :].

上述的操作e.g. p.loc['a'] is equivalent to p.loc['a', :], 二者是等价的.

基本使用

As mentioned when introducing the data structures in the last section, the primary function of indexing with [] (a.k.a. __getitem__ for those familiar with implementing class behavior in Python) is selecting out lower-dimensional slices. The following table shows return type values when indexing pandas objects with []:

Here we construct a simple time series data set to use for illustrating the indexing functionality:

Note

None of the indexing functionality is time series specific unless specifically stated.

除非特别说明 否则没有索引功能是特定于时间序列的.

Thus, as per above, we have the most basic indexing using []:

因此, 上述内容, 最为基本得操作是使用[].

You can pass a list of columns to [] to select columns in that order. If a column is not contained in the DataFrame, an exception will be raised. Multiple columns can also be set in this manner:

You may find this useful for applying a transform (in-place) to a subset of the columns.

你也许会发现这对于转置数据非常有用.

Warning

pandas aligns all AXES when setting Series and DataFrame from .loc, and .iloc.

pandas允许所有的轴设置series或者dataframe, 当使用.loc, and .iloc

This will not modify df because the column alignment is before value assignment.

这不会修改dataframe, 因为列对其实在赋值之前.(这句话说的是什么?)

The correct way to swap column values is by using raw values:

交换两列数据的方式

You may access an index on a Series or column on a DataFrame directly as an attribute:

属性操作符访问(逗号)

Warning

If you are using the IPython environment, you may also use tab-completion to see these accessible attributes.

You can also assign a dict to a row of a DataFrame:

通过字典的方式对其中的行数据进行修改

You can use attribute access to modify an existing element of a Series or column of a DataFrame, but be careful; if you try to use attribute access to create a new column, it creates a new attribute rather than a new column. In 0.21.0 and later, this will raise a UserWarning:

切片范围

The most robust and consistent way of slicing ranges along arbitrary axes is described in the Selection by Position section detailing the .iloc method. For now, we explain the semantics of slicing using the [] operator.

With Series, the syntax works exactly as with an ndarray, returning a slice of the values and the corresponding labels:

Note that setting works as well:

With DataFrame, slicing inside of [] slices the rows. This is provided largely as a convenience since it is such a common operation.

基于标签

Warning

Whether a copy or a reference is returned for a setting operation, may depend on the context. This is sometimes called chained assignment and should be avoided. See Returning a View versus Copy.

注意链式索引操作

Warning

.loc is strict when you present slicers that are not compatible (or convertible) with the index type. For example using integers in a DatetimeIndex. These will raise a TypeError.

.loc是严格区分数据类型的, 假如输入的数据类型和当前的索引的数据类型不一致将直接报错

String likes in slicing can be convertible to the type of the index and lead to natural slicing.

Warning

Changed in version 1.0.0.

pandas will raise a KeyError if indexing with a list with missing labels. See list-like Using loc with missing keys in a list is Deprecated.

pandas会报键值错误, 对不存在的标签进行索引.

pandas provides a suite of methods in order to have purely label based indexing. This is a strict inclusion based protocol. Every label asked for must be in the index, or a KeyError will be raised. When slicing, both the start bound AND the stop bound are included, if present in the index. Integers are valid labels, but they refer to the label and not the position.

The .loc attribute is the primary access method. The following are valid inputs:

Note that setting works as well:

注意这种操作对于赋值也是可行的

With a DataFrame:

Accessing via label slices:

For getting a cross section using a label (equivalent to df.xs('a')):

等价操作

For getting values with a boolean array:

使用布尔数组取值

NA values in a boolean array propagate as False:

NA值会被视作False

Changed in version 1.0.2.

For getting a value explicitly:

When using .loc with slices, if both the start and the stop labels are present in the index, then elements located between the two (including them) are returned:

If at least one of the two is absent, but the index is sorted, and can be compared against start and stop labels, then slicing will still work as expected, by selecting labels which rank between the two:

如果传入标签在index上是存在缺失的, 假如索引被排序过(或者是有序的index), 将对比于起始和结尾标签, 将返回对比后的内容.

注意这里的特性.

不管是数字类型的index还是字符串类型的index, 都满足只要是有序的, 均可以如此访问.

However, if at least one of the two is absent and the index is not sorted, an error will be raised (since doing otherwise would be computationally expensive, as well as potentially ambiguous for mixed type indexes). For instance, in the above example, s.loc[1:6] would raise KeyError.

缺失的index或者是无序的index, 都会引发错误. 这样做是非常耗费资源的, 以及潜在的问题, 在多级的index中.

For the rationale behind this behavior, see Endpoints are inclusive.

终点包含的操作

Also, if the index has duplicate labels and either the start or the stop label is duplicated, an error will be raised. For instance, in the above example, s.loc[2:5] would raise a KeyError.

index中存在相同的label将触发错误, 当传入的索引label包含有这重复的label.

For more information about duplicate labels, see Duplicate Labels.

基于位置

Warning

Whether a copy or a reference is returned for a setting operation, may depend on the context. This is sometimes called chained assignment and should be avoided. See Returning a View versus Copy.

pandas provides a suite of methods in order to get purely integer based indexing. The semantics follow closely Python and NumPy slicing. These are 0-based indexing. When slicing, the start bound is included, while the upper bound is excluded. Trying to use a non-integer, even a valid label will raise an IndexError.

The .iloc attribute is the primary access method. The following are valid inputs:

Note that setting works as well:

With a DataFrame:

Select via integer slicing:

Select via integer list:

For getting a cross section using an integer position (equiv to df.xs(1)):

Out of range slice indexes are handled gracefully just as in Python/NumPy.

超出范围处理, 起点和终点超出范围, 返回空值, 终点超出范围, 起点开始截取到最后的位置.

Note that using slices that go out of bounds can result in an empty axis (e.g. an empty DataFrame being returned).

A single indexer that is out of bounds will raise an IndexError. A list of indexers where any element is out of bounds will raise an IndexError.

单个索引的情况下(含多个索引值列表, 即除了切片范围的情况下), 超出范围将导致错误.

基于函数

.loc, .iloc, and also [] indexing can accept a callable as indexer. The callable must be a function with one argument (the calling Series or DataFrame) that returns valid output for indexing.

实际上就是函数返回值而已

You can use callable indexing in Series.

Using these methods / indexers, you can chain data selection operations without using a temporary variable.

If you wish to get the 0th and the 2nd elements from the index in the ‘A’ column, you can do:

This can also be expressed using .iloc, by explicitly getting locations on the indexers, and using positional indexing to select things.

For getting multiple indexers, using .get_indexer:

注意使用不存在的标签进行索引, 这种方法将被逐步废弃.

Warning

Changed in version 1.0.0.

Using .loc or [] with a list with one or more missing labels will no longer reindex, in favor of .reindex.

In prior versions, using .loc[list-of-labels] would work as long as at least 1 of the keys was found (otherwise it would raise a KeyError). This behavior was changed and will now raise a KeyError if at least one label is missing. The recommended alternative is to use .reindex().

当传入的索引标签列中只要存在有一个不存在的标签, 就会出现keyerror的提示, 并推荐使用reindex()(即对替换为新的索引)

For example.

Selection with all keys found is unchanged.

Previous behavior

未改版之前, 是返回一个NaN值填充超出范围的数据

Current behavior

注意改版之后的变化, 多个index, 其中有不存在的, 将直接报错

重置索引

注意不是reset_index()

Series.reindex(*args, **kwargs)[source]

Conform Series to new index with optional filling logic.

Places NA/NaN in locations having no value in the previous index. A new object is produced unless the new index is equivalent to the current one and copy=False.

The idiomatic way to achieve selecting potentially not-found elements is via .reindex(). See also the section on reindexing.

对于索引不存在的值, 常规方式是使用reindex()来取值, 这种方式会对空值进行填充而不是直接报错.

这种方式实际这不算取值, 这是折中方案.

Alternatively, if you want to select only valid keys, the following is idiomatic and efficient; it is guaranteed to preserve the dtype of the selection.

可选方案, 可以使用index.intersection

Form the intersection of two Index objects.

This returns a new Index with elements common to the index and other.

取得二者的交集索引

Having a duplicated index will raise for a .reindex():

当存在重复值是, reindex()将报错.

Generally, you can intersect the desired labels with the current axis, and then reindex.

However, this would still raise if your resulting index is duplicated.

虽然使用了索引交集, 但是存在重复的label依然还是会报错.

随机样本的选取

A random selection of rows or columns from a Series or DataFrame with the sample() method. The method will sample rows by default, and accepts a specific number of rows/columns to return, or a fraction of rows.

By default, sample will return each row at most once, but one can also sample with replacement using the replace option:

默认每一行随机样本只会返回一次, 需要多次, 可以设置参数replace.

By default, each row has an equal probability of being selected, but if you want rows to have different probabilities, you can pass the sample function sampling weights as weights. These weights can be a list, a NumPy array, or a Series, but they must be of the same length as the object you are sampling. Missing values will be treated as a weight of zero, and inf values are not allowed. If weights do not sum to 1, they will be re-normalized by dividing all weights by the sum of the weights. For example:

默认状态下每一行数据的随机可能性是相同的, 但是可以手动制定被选中的几率权重.

When applied to a DataFrame, you can use a column of the DataFrame as sampling weights (provided you are sampling rows and not columns) by simply passing the name of the column as a string.

也可以制定列的权重.

sample also allows users to sample columns instead of rows using the axis argument.

Finally, one can also set a seed for sample’s random number generator using the random_state argument, which will accept either an integer (as a seed) or a NumPy RandomState object.

扩增

The .loc/[] operations can perform enlargement when setting a non-existent key for that axis.

.loc[]在设置一个不存在的索引(标签)时, 会自动执行扩增.

In the Series case this is effectively an appending operation.

在series中, 这就像append一样.

A DataFrame can be enlarged on either axis via .loc.

在dataframe中也一样

This is like an append operation on the DataFrame.

更快的操作方式, at and iat, 适用于单个值的访问和设置.

Since indexing with [] must handle a lot of cases (single-label access, slicing, boolean indexing, etc.), it has a bit of overhead in order to figure out what you’re asking for. If you only want to access a scalar value, the fastest way is to use the at and iat methods, which are implemented on all of the data structures.

Similarly to loc, at provides label based scalar lookups, while, iat provides integer based lookups analogously to iloc

You can also set using these same indexers.

你也可以像使用坐标一样.

at may enlarge the object in-place as above if the indexer is missing.

at操作, 在索引不存在的情况下, 将自动扩增dataframe

布尔索引

Another common operation is the use of boolean vectors to filter the data. The operators are: | for or, & for and, and ~ for not. These must be grouped by using parentheses, since by default Python will evaluate an expression such as df['A'] > 2 & df['B'] < 3 as df['A'] > (2 & df['B']) < 3, while the desired evaluation order is (df['A'] > 2) & (df['B'] < 3).

多个布尔值对应的操作符为: | for or, & for and, and ~ for not

Using a boolean vector to index a Series works exactly as in a NumPy ndarray:

You may select rows from a DataFrame using a boolean vector the same length as the DataFrame’s index (for example, something derived from one of the columns of the DataFrame):

List comprehensions and the map method of Series can also be used to produce more complex criteria:

适用于更为复杂的筛选, 在map等函数中.

With the choice methods Selection by Label, Selection by Position, and Advanced Indexing you may select along more than one axis using boolean vectors combined with other indexing expressions.

混合使用, 同时使用其他的取值方式, 如切片.

Warning

iloc supports two kinds of boolean indexing. If the indexer is a boolean Series, an error will be raised. For instance, in the following example, df.iloc[s.values, 1] is ok. The boolean indexer is an array. But df.iloc[s, 1] would raise ValueError.

需要注意, iloc支持两种布尔索引, 假如执行的索引是一个序列的布尔索引, 将直接报错, 需要取出其中的值来执行.

isin执行索引

Consider the isin() method of

Series, which returns a boolean vector that is true wherever the Series elements exist in the passed list. This allows you to select rows where one or more columns have values you want:

The same method is available for Index objects and is useful for the cases when you don’t know which of the sought labels are in fact present:

In addition to that, MultiIndex allows selecting a separate level to use in the membership check:

DataFrame also has an isin() method. When calling isin, pass a set of values as either an array or dict. If values is an array, isin returns a DataFrame of booleans that is the same shape as the original DataFrame, with True wherever the element is in the sequence of values.

Oftentimes you’ll want to match certain values with certain columns. Just make values a dict where the key is the column, and the value is a list of items you want to check for.

To return the DataFrame of booleans where the values are not in the original DataFrame, use the ~ operator:

执行 ~ 非操作.

Combine DataFrame’s isin with the any() and all() methods to quickly select subsets of your data that meet a given criteria. To select a row where each column meets its own criterion:

和any和all一起搭配使用.

where()的使用

Selecting values from a Series with a boolean vector generally returns a subset of the data. To guarantee that selection output has the same shape as the original data, you can use the where method in Series and DataFrame.

where()的操作保证返回的数据内容大小和series一致.

To return only the selected rows:

To return a Series of the same shape as the original:

Selecting values from a DataFrame with a boolean criterion now also preserves input data shape. where is used under the hood as the implementation. The code below is equivalent to df.where(df < 0).

在df中则等价于[]的操作.

In addition, where takes an optional other argument for replacement of values where the condition is False, in the returned copy.

额外的参数提供自动填充的功能, 对返回false的项进行数据填充

You may wish to set values based on some boolean criteria. This can be done intuitively like so:

By default, where returns a modified copy of the data. There is an optional parameter inplace so that the original data can be modified without creating a copy:

Note

The signature for DataFrame.where() differs from numpy.where(). Roughly df1.where(m, df2) is equivalent to np.where(m, df1, df2).

Alignment

Furthermore, where aligns the input boolean condition (ndarray or DataFrame), such that partial selection with setting is possible. This is analogous to partial setting via .loc (but on the contents rather than the axis labels).

Where can also accept axis and level parameters to align the input when performing the where.

This is equivalent to (but faster than) the following.

where can accept a callable as condition and other arguments. The function must be with one argument (the calling Series or DataFrame) and that returns valid output as condition and other argument.

mask() is the inverse boolean operation of where.

这个函式是逆布尔操作.

使用numpy进行扩增.

An alternative to where() is to use numpy.where(). Combined with setting a new column, you can use it to enlarge a DataFrame where the values are determined conditionally.

对于where()函数的备选np.where()

Consider you have two choices to choose from in the following DataFrame. And you want to set a new column color to ‘green’ when the second column has ‘Z’. You can do the following:

If you have multiple conditions, you can use numpy.select() to achieve that. Say corresponding to three conditions there are three choice of colors, with a fourth color as a fallback, you can do the following.

搭配numpy.select()

类似于sql语句的执行

DataFrame objects have a query() method that allows selection using an expression.

You can get the value of the frame where column b has values between the values of columns a and c. For example:

Do the same thing but fall back on a named index if there is no column with the name a.

If instead you don’t want to or cannot name your index, you can use the name index in your query expression:

Note

If the name of your index overlaps with a column name, the column name is given precedence. For example,

注意这里, 假如对索引进行命名, 出现覆盖了列的名称的情况, query优先使用的是索引的名称

You can still use the index in a query expression by using the special identifier ‘index’:

但是可以使用index来表示索引(虽然索引已经变更名称)

If for some reason you have a column named index, then you can refer to the index as ilevel_0 as well, but at this point you should consider renaming your columns to something less ambiguous.

注意, 假如名称中存在index的列, 可以使用ilevel_0来表示index

多索引的情形

You can also use the levels of a DataFrame with a MultiIndex as if they were columns in the frame:

Of course, expressions can be arbitrarily complex too:

query()执行效率

DataFrame.query() using numexpr is slightly faster than Python for large frames.

Note

You will only see the performance benefits of using the numexpr engine with DataFrame.query() if your frame has more than approximately 200,000 rows.

大概在超过20万行数据, 这种差异更为明显

This plot was created using a DataFrame with 3 columns each containing floating point values generated using numpy.random.randn().

重复的数据

If you want to identify and remove duplicate rows in a DataFrame, there are two methods that will help: duplicated and drop_duplicates. Each takes as an argument the columns to use to identify duplicated rows.

By default, the first observed row of a duplicate set is considered unique, but each method has a keep parameter to specify targets to be kept.

Also, you can pass a list of columns to identify duplications.

To drop duplicates by index value, use Index.duplicated then perform slicing. The same set of options are available for the keep parameter.

类字典的取值方式

Each of Series or DataFrame have a get method which can return a default value.

每个series和df都有一个get方法进行取值, 和字典类似, 也可以定义一个默认值

Sometimes you want to extract a set of values given a sequence of row labels and column labels, this can be achieved by pandas.factorize and NumPy indexing. For instance:

This method is useful for obtaining a numeric representation of an array when all that matters is identifying distinct values. factorize is available as both a top-level function [pandas.factorize()](https://pandas.pydata.org/docs/reference/api/pandas.factorize.html?highlight=pd factorize#pandas.factorize), and as a method Series.factorize() and Index.factorize().

Formerly this could be achieved with the dedicated DataFrame.lookup method which was deprecated in version 1.2.0.

lookup方法将逐步废弃, 自1.2.0版本.

索引对象

The pandas Index class and its subclasses can be viewed as implementing an ordered multiset. Duplicates are allowed. However, if you try to convert an Index object with duplicate entries into a set, an exception will be raised.

索引可以设置重复值, 但是在转换带有重复值索引到set时会触发异常.

Index also provides the infrastructure necessary for lookups, data alignment, and reindexing. The easiest way to create an Index directly is to pass a list or other sequence to Index:

You can also pass a name to be stored in the index:

The name, if set, will be shown in the console display:

设置元数据

Indexes are " mostly immutable" , but it is possible to set and change their name attribute. You can use the rename, set_names to set these attributes directly, and they default to returning a copy.

索引对象通常是不变得, 但是有时会改变name属性.

rename, set_names默认返回新得副本.

See Advanced Indexing for usage of MultiIndexes.

set_names, set_levels, and set_codes also take an optional level argument

set_names, set_levels, and set_codes均支持level参数.

索引对象的set操作.

The two main operations are union and intersection. Difference is provided via the .difference() method.

两个主要的操作符是union和intersection, difference()提供差异的数值.

Also available is the symmetric_difference operation, which returns elements that appear in either idx1 or idx2, but not in both. This is equivalent to the Index created by idx1.difference(idx2).union(idx2.difference(idx1)), with duplicates dropped.

symmetric_difference, 删除掉相同值, 只保留二者差异的部分, 等价于 idx1.difference(idx2).union(idx2.difference(idx1))

Important

Even though Index can hold missing values (NaN), it should be avoided if you do not want any unexpected results. For example, some operations exclude missing values implicitly.

注意索然索引会处理空值, 但是空值的存在会导致不确定的结果.

例如一些操作可能会隐式忽略丢失的空值(例如sum()).

处理空值

Important

Even though Index can hold missing values (NaN), it should be avoided if you do not want any unexpected results. For example, some operations exclude missing values implicitly.

Index.fillna fills missing values with specified scalar value.

填充丢失的空值

设置/重置索引

Occasionally you will load or create a data set into a DataFrame and want to add an index after you’ve already done so. There are a couple of different ways.

设置索引

DataFrame has a set_index() method which takes a column name (for a regular Index) or a list of column names (for a MultiIndex). To create a new, re-indexed DataFrame:

The append keyword option allow you to keep the existing index and append the given columns to a MultiIndex:

append参数, 添加索引到现有索引上.

Other options in set_index allow you not drop the index columns or to add the index in-place (without creating a new object):

inplace参数, 在原对象上进行.

重置索引(不同于reindex)

As a convenience, there is a new function on DataFrame called reset_index() which transfers the index values into the DataFrame’s columns and sets a simple integer index. This is the inverse operation of set_index().

和set_index()进行的是相反的操作, 注意撤销掉索引之后, 索引的列重新放置的位置.

The output is more similar to a SQL table or a record array. The names for the columns derived from the index are the ones stored in the names attribute.

You can use the level keyword to remove only a portion of the index:

多层的索引, 可以制定一个level参数来控制重置的索引.

reset_index takes an optional parameter drop which if true simply discards the index, instead of putting index values in the DataFrame’s columns.

支持一个可选参数drop, 如果设置为True, 则将撤销索引(而不会将扯些索引重新添加到新的列上, 这种事默认的操作)

增加一个 ad hoc索引

If you create an index yourself, you can just assign it to the index field:

(这部分的内容见另一篇更为详细的翻译.)

When setting values in a pandas object, care must be taken to avoid what is called chained indexing. Here is an example.

Compare these two access methods:

These both yield the same results, so which should you use? It is instructive to understand the order of operations on these and why method 2 (.loc) is much preferred over method 1 (chained []).

dfmi['one'] selects the first level of the columns and returns a DataFrame that is singly-indexed. Then another Python operation dfmi_with_one['second'] selects the series indexed by 'second'. This is indicated by the variable dfmi_with_one because pandas sees these operations as separate events. e.g. separate calls to __getitem__, so it has to treat them as linear operations, they happen one after another.

Contrast this to df.loc[:,('one','second')] which passes a nested tuple of (slice(None),('one','second')) to a single call to __getitem__. This allows pandas to deal with this as a single entity. Furthermore this order of operations can be significantly faster, and allows one to index both axes if so desired.

链式操作赋值为什么失败?

The problem in the previous section is just a performance issue. What’s up with the SettingWithCopy warning? We don’t usually throw warnings around when you do something that might cost a few extra milliseconds!

在上一章这个只是一个执行效率的问题, 但是pandas不会为了那么一丁点事件的差异而发出一个警告.

But it turns out that assigning to the product of chained indexing has inherently unpredictable results. To see this, think about how the Python interpreter executes this code:

这是因为链式操作可能产生的不可预测的结果.这需要从python的解析器执行的具体代码来查看问题.

But this code is handled differently:

代码上处理的差异

See that __getitem__ in there? Outside of simple cases, it’s very hard to predict whether it will return a view or a copy (it depends on the memory layout of the array, about which pandas makes no guarantees), and therefore whether the __setitem__ will modify dfmi or a temporary object that gets thrown out immediately afterward. That’s what SettingWithCopy is warning you about!

注意__getitem__的位置, 这一步的操作很难预测到底是返回视图还是一个副本(这可能取决于数组内存的占用, 这不是pandas可以控制的). 然后才执行__setitem__, 这就会马上触发异常警告.

Note

You may be wondering whether we should be concerned about the loc property in the first example. But dfmi.loc is guaranteed to be dfmi itself with modified indexing behavior, so dfmi.loc.__getitem__ / dfmi.loc.__setitem__ operate on dfmi directly. Of course, dfmi.loc.__getitem__(idx) may be a view or a copy of dfmi.

Sometimes a SettingWithCopy warning will arise at times when there’s no obvious chained indexing going on. These are the bugs that SettingWithCopy is designed to catch! pandas is probably trying to warn you that you’ve done this:

注意, 有时这个警告的触发并不是明显的链式操作中. 这是pandas的设计机制, 旨在警告用户曾试图发起类似的操作.

Yikes!

对于警告的处理方式.

When you use chained indexing, the order and type of the indexing operation partially determine whether the result is a slice into the original object, or a copy of the slice.

pandas has the SettingWithCopyWarning because assigning to a copy of a slice is frequently not intentional, but a mistake caused by chained indexing returning a copy where a slice was expected.

If you would like pandas to be more or less trusting about assignment to a chained indexing expression, you can set the option mode.chained_assignment to one of these values:

This however is operating on a copy and will not work.

A chained assignment can also crop up in setting in a mixed dtype frame.

Note

These setting rules apply to all of .loc/.iloc.

The following is the recommended access method using .loc for multiple items (using mask) and a single item using a fixed index:

The following can work at times, but it is not guaranteed to, and therefore should be avoided:

Last, the subsequent example will not work at all, and so should be avoided:

Warning

The chained assignment warnings / exceptions are aiming to inform the user of a possibly invalid assignment. There may be false positives; situations where a chained assignment is inadvertently reported.