**Pandas**

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**Data Selection and Renaming**

# Pandas DataFrame | query method

*schedule*Mar 5, 2023

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Pandas' `DataFrame.query(~)`

method filters rows according to the provided boolean expression.

The `query(~)`

method returns a copy of the data, so you cannot use the method to update values of the DataFrame. To perform updates, use properties like `loc`

and `iloc`

, which returns references instead of copies.

# Parameters

1. `expr`

| `string`

The criteria by which to filter the rows. The string must be a boolean expression (e.g. `"A == 2"`

and `"A > 3"`

where `A`

is a column name).

2. `inplace`

| `boolean`

| `optional`

If

`True`

, then the method will directly modify the source DataFrame instead of creating a new DataFrame.If

`False`

, then a new DataFrame will be created and returned.

By default, `inplace=False`

.

# Return Value

A `DataFrame`

containing the filtered rows.

# Examples

Consider the following DataFrame:

```
A B C0 1 4 71 2 5 82 3 6 9
```

## Equality queries

To get all rows where the value for column `A`

is `2`

:

```
df.query("A == 2")
A B C1 2 5 8
```

For a not-equals query, use `!=`

instead.

## Queries with string values

When referring to string-typed values, wrap them the values in `'`

or `"`

like so:

```
df.query("A == 'a'")
A0 a
```

## Range queries

To get all rows where the value for column `A`

is greater than `1`

:

```
df.query("A > 1")
A B C1 2 5 82 3 6 9
```

## Interval queries

To get all rows where the value for column `A`

is between 1 and 3 (both ends exclusive):

```
df.query("1 < A < 3")
A B C1 2 5 8
```

## AND queries

To get all rows where column `A == 3`

and column `C == 9`

:

```
df.query("A == 3 and C == 9")
A B C2 3 6 9
```

## OR queries

To get all rows where column `A == 1`

or column `C == 9`

:

```
df.query("A == 1 or C == 9")
A B C0 1 4 72 3 6 9
```

## Queries with column-comparisons

To get all rows where the value for column `C`

is greater than that for column `A`

:

```
df.query("C > A")
A B C0 1 4 71 2 5 82 3 6 9
```

## Queries with arithmetic operations

To get all rows where the value for `C`

plus 1 is 10:

```
df.query("C + 1 == 10")
A B C2 3 6 9
```

## Queries with object access

You can also access object properties and methods using the dot notation:

```
df.query("B.values.max() < C")
A B C0 1 4 71 2 5 82 3 6 9
```

Here, note the following:

`B`

is of type`Series`

, and so you have access to all the properties and methods available for`Series`

.One such property is

`values`

, which is simply a`Numpy array`

representation of the`Series`

.Next, we are calling

`max()`

, which is a method available to all Numpy arrays. In this case,`max()`

returns`6`

since`6`

is the largest value in column`B`

.

## IN queries

Consider the following DataFrame:

```
df
A B0 aa b1 aa b2 aA b3 AA b
```

To get all rows where the value for `A`

is in `arr`

:

```
arr = ["aa","AA"]df.query("A in @arr")
A B0 aa b1 aa b3 AA b
```

Here, the `arr`

is prefixed with `@`

since it is an external variable.

## NOT IN queries

Consider the same DataFrame:

```
df
A B0 aa b1 aa b2 aA b3 AA b
```

To get all rows where the value for `A`

is not in `arr`

:

```
arr = ["aa","AA"]df.query("A not in @arr")
A B2 aA b
```

## Referencing the index

Consider the following DataFrame:

```
df
A B0 2 51 3 62 4 7
```

To refer to the index, simply include the word `index`

like so:

```
df.query("index > 1")
A B2 4 7
```

## Handling column names with spaces

When your column names contain spaces, you need to add the back-ticks ```

around the column name.

Consider the following DataFrame:

```
df
A A B0 1 31 2 4
```

Here, the first column (i.e. `A A`

) contains a space in its name. To deal with this, wrap ```

like follows:

```
df.query("`A A` == 2")
A A B1 2 4
```