` `

Expressions¶

The Expression string builder
List of functions
Function Editor

Based on layer data and prebuilt or user defined functions, Expressions offer a powerful way to manipulate attribute value, geometry and variables in order to dynamically change the geometry style, the content or position of the label, the value for diagram, the height of a composer item, select some features, create virtual field ...

The Expression string builder ¶

Main dialog to build expressions, the Expression string builder is available from many parts in QGIS and, can particularly be accessed when:

clicking the button;
selecting features with the ^{Select By Expression...} tool;
editing attributes with e.g. the ^{Field calculator} tool;
manipulating symbology, label or composer item parameters with the ^{Data defined override} tool (see Data defined override setup);
building a geometry generator symbol layer;
doing some geoprocessing.

The Expression builder dialog offers access to the:

Expression tab which, thanks to a list of predefined functions, helps to write and check the expression to use;
Function Editor tab which helps to extend the list of functions by creating custom ones.

Some use cases of expressions:

From Field Calculator, calculate a “pop_density” field using existing “total_pop” and “area_km2” fields:
```
"total_pop" / "area_km2"
```

Update the field “density_level” with categories according to the “pop_density” values:

CASE WHEN "pop_density" < 50 THEN 'Low population density'
     WHEN "pop_density" >= 50 and "pop_density" < 150 THEN 'Medium population density'
     WHEN "pop_density" >= 150 THEN 'High population density'
END

Apply a categorized style to all the features according to whether their average house price is smaller or higher than 10000€ per square metre:
```
"price_m2" > 10000
```
Using the “Select By Expression...” tool, select all the features representing areas of “High population density” and whose average house price is higher than 10000€ per square metre:
```
"density_level" = 'High population density' and "price_m2" > 10000
```
Likewise, the previous expression could also be used to define which features should be labeled or shown in the map.

Using expressions offers you a lot of possibilities.

Tip

Use named parameters to improve the expression reading

Some functions require many parameters to be set. The expression engine supports the use of named parameters. This means that instead of writing the cryptic expression clamp( 1, 2, 9), you can use clamp( min:=1, value:=2, max:=9). This also allows arguments to be switched, e.g. clamp( value:=2, max:=9, min:=1). Using named parameters helps clarify what the arguments for an expression function refer to, which is helpful when you are trying to interpret an expression at a later date!

List of functions ¶

The Expression tab provides the main interface to write expressions using functions, layer’s fields and values. It contains widgets to:

type expressions using functions and/or fields. At the bottom of the dialog, is displayed the result of the expression evaluated on the first feature of the layer.
select the appropriate function among a list, organized in groups. A search box is available to filter the list and quickly find a particular function or field. Double-clicking on the item’s name adds it to the expression being written.
display help for each function selected. When a field is selected, this widget shows a sample of its values. Double-clicking a value adds it to the expression.

The Expression tab

Operators ¶

This group contains operators (e.g., +, -, *). Note that for most of the mathematical functions below, if one of the inputs is NULL then the result is NULL.

Function	Description
a + b	Addition of two values (a plus b)
a - b	Subtraction of two values (a minus b).
a * b	Multiplication of two values (a multiplied by b)
a / b	Division of two values (a divided by b)
a % b	Remainder of division of a by b (eg, 7 % 2 = 1, or 2 fits into 7 three times with remainder 1)
a ^ b	Power of two values (for example, 2^2=4 or 2^3=8)
a < b	Compares two values and evaluates to 1 if the left value is less than the right value (a is smaller than b)
a <= b	Compares two values and evaluates to 1 if the left value isless than or equal to the right value
a <> b	Compares two values and evaluates to 1 if they are not equal
a = b	Compares two values and evaluates to 1 if they are equal
a != b	a and b are not equal
a > b	Compares two values and evaluates to 1 if the left value is greater than the right value (a is larger than b)
a >= b	Compares two values and evaluates to 1 if the left value is greater than or equal to the right value
a ~ b	a matches the regular expression b
\|\|	Joins two values together into a string. If one of the values is NULL the result will be NULL
‘\n’	Inserts a new line in a string
LIKE	Returns 1 if the first parameter matches the supplied pattern
ILIKE	Returns 1 if the first parameter matches case-insensitive the supplied pattern (ILIKE can be used instead of LIKE to make the match case-insensitive)
a IS b	Tests whether two values are identical. Returns 1 if a is the same as b
a OR b	Returns 1 when condition a or condition b is true
a AND b	Returns 1 when conditions a and b are true
NOT	Negates a condition
column name “column name”	Value of the field column name, take care to not be confused with simple quote, see below
‘string’	a string value, take care to not be confused with double quote, see above
NULL	null value
a IS NULL	a has no value
a IS NOT NULL	a has a value
a IN (value[,value])	a is below the values listed
a NOT IN (value[,value])	a is not below the values listed

Note

About fields concatenation

You can concatenate strings using either || or +. The latter also means sum up expression. So if you have an integer (field or numeric value) this can be error prone. In this case, you should use ||. If you concatenate two string values, you can use both.

Some examples:

Joins a string and a value from a column name:

'My feature''s id is: ' || "gid"
'My feature''s id is: ' + "gid" => triggers an error as gid is an integer
"country_name" + '(' + "country_code" + ')'
"country_name" || '(' || "country_code" || ')'

Test if the “description” attribute field starts with the ‘Hello’ string in the value (note the position of the % character):
```
"description" LIKE 'Hello%'
```

Conditionals ¶

This group contains functions to handle conditional checks in expressions.

Function	Description
CASE WHEN ... THEN ... END	Evaluates an expression and returns a result if true. You can test multiple conditions
CASE WHEN ... THEN ... ELSE ... END	Evaluates an expression and returns a different result whether it’s true or false. You can test multiple conditions
coalesce	Returns the first non-NULL value from the expression list
if	Tests a condition and returns a different result depending on the conditional check
regexp_match	Returns true if any part of a string matches the supplied regular expression

Some example:

Send back a value if the first condition is true, else another value:
```
CASE WHEN "software" LIKE '%QGIS%' THEN 'QGIS' ELSE 'Other' END
```

Mathematical Functions ¶

This group contains math functions (e.g., square root, sin and cos).

Function	Description
abs	Returns the absolute value of a number
acos	Returns the inverse cosine of a value in radians
asin	Returns the inverse sine of a value in radians
atan	Returns the inverse tangent of a value in radians
atan2(y,x)	Returns the inverse tangent of y/x by using the signs of the two arguments to determine the quadrant of the result
azimuth(a,b)	Returns the north-based azimuth as the angle in radians measured clockwise from the vertical on point a to point b
ceil	Rounds a number upwards
clamp	Restricts an input value to a specified range
cos	Returns the cosine of a value in radians
degrees	Converts from radians to degrees
exp	Returns exponential of a value
floor	Rounds a number downwards
ln	Returns the natural logarithm of the passed expression
log	Returns the value of the logarithm of the passed value and base
log10	Returns the value of the base 10 logarithm of the passed expression
max	Returns the largest value in a set of values
min	Returns the smallest value in a set of values
pi	Returns the value of pi for calculations
radians	Converts from degrees to radians
rand	Returns the random integer within the range specified by the minimum and maximum argument (inclusive)
randf	Returns the random float within the range specified by the minimum and maximum argument (inclusive)
round	Rounds to number of decimal places
scale_exp	Transforms a given value from an input domain to an output range using an exponential curve
scale_linear	Transforms a given value from an input domain to an output range using linear interpolation
sin	Returns the sine of an angle
sqrt	Returns the square root of a value
tan	Returns the tangent of an angle

Aggregates Functions ¶

This group contains functions which aggregate values over layers and fields.

Function	Description
aggregate	Returns an aggregate value calculated using features from another layer
concatenate	Returns the all aggregated strings from a field or expression joined by a delimiter
count	Returns the count of matching features
count_distinct	Returns the count of distinct values
count_missing	Returns the count of missing (null) values
iqr	Returns the calculated inter quartile range from a field or expression
majority	Returns the aggregate majority of values (most commonly occurring value) from a field or expression
max_length	Returns the maximum length of strings from a field or expression
maximum	Returns the aggregate maximum value from a field or expression
mean	Returns the aggregate mean value from a field or expression
median	Returns the aggregate median value from a field or expression
min_length	Returns the minimum length of strings from a field or expression
minimum	Returns the aggregate minimum value from a field or expression
minority	Returns the aggregate minority of values (least commonly occurring value) from a field or expression
q1	Returns the calculated first quartile from a field or expression
q3	Returns the calculated third quartile from a field or expression
range	Returns the aggregate range of values (maximum - minimum) from a field or expression
relation_aggregate	Returns an aggregate value calculated using all matching child features from a layer relation
stdev	Returns the aggregate standard deviation value from a field or expression
sum	Returns the aggregate summed value from a field or expression

Examples:

Return the maximum of the “passengers” field from features in the layer grouped by “station_class” field:
```
maximum("passengers", group_by:="station_class")
```
Calculate the total number of passengers for the stations inside the current atlas feature:
```
aggregate('rail_stations','sum',"passengers",
  intersects(@atlas_geometry, $geometry))
```

Return the mean of the “field_from_related_table” field for all matching child features using the ‘my_relation’ relation from the layer:

aggregate_relation('my_relation', 'mean', "field_from_related_table")

or:

aggregate_relation(relation:='my_relation', calculation := 'mean',
  expression := "field_from_related_table")

Color Functions ¶

This group contains functions for manipulating colors.

Function	Description
color_cmyk	Returns a string representation of a color based on its cyan, magenta, yellow and black components
color_cmyka	Returns a string representation of a color based on its cyan, magenta, yellow, black and alpha (transparency) components
color_hsl	Returns a string representation of a color based on its hue, saturation, and lightness attributes
color_hsla	Returns a string representation of a color based on its hue, saturation, lightness and alpha (transparency) attributes
color_hsv	Returns a string representation of a color based on its hue, saturation, and value attributes
color_hsva	Returns a string representation of a color based on its hue, saturation, value and alpha (transparency) attributes
color_part	Returns a specific component from a color string, eg the red component or alpha component
color_rgb	Returns a string representation of a color based on its red, green, and blue components
color_rgba	Returns a string representation of a color based on its red, green, blue, and alpha (transparency) components
darker	Returns a darker (or lighter) color string
lighter	Returns a lighter (or darker) color string
project_color	Returns a color from the project’s color scheme
ramp_color	Returns a string representing a color from a color ramp
set_color_part	Sets a specific color component for a color string, eg the red component or alpha component

Conversions Functions ¶

This group contains functions to convert one data type to another (e.g., string to integer, integer to string).

Function	Description
to_date	Converts a string into a date object
to_datetime	Converts a string into a datetime object
to_int	Converts a string to integer number
to_interval	Converts a string to an interval type (can be used to take days, hours, months, etc. of a date)
to_real	Converts a string to a real number
to_string	Converts number to string
to_time	Converts a string into a time object

Custom Functions ¶

This group contains functions created by the user. See Function Editor for more details.

Date and Time Functions ¶

This group contains functions for handling date and time data.

Function	Description
age	Returns as an interval the difference between two dates or datetimes
day	Extracts the day from a date or datetime, or the number of days from an interval
day_of_week	Returns a number corresponding to the day of the week for a specified date or datetime
hour	Extracts the hour from a datetime or time, or the number of hours from an interval
minute	Extracts the minute from a datetime or time, or the number of minutes from an interval
month	Extracts the month part from a date or datetime, or the number of months from an interval
now	Returns current date and time
second	Extracts the second from a datetime or time, or the number of seconds from an interval
week	Extracts the week number from a date or datetime, or the number of weeks from an interval
year	Extracts the year part from a date or datetime, or the number of years from an interval

This group also shares several functions with the Conversions Functions ( to_date, to_time, to_datetime, to_interval) and String Functions (format_date) groups.

Some examples:

Get today’s month and year in the “month_number/year” format:
```
format_date(now(),'MM/yyyy')
-- Returns '03/2017'
```

Besides these functions, subtracting dates, datetimes or times using the - (minus) operator will return an interval.

Adding or subtracting an interval to dates, datetimes or times, using the + (plus) and - (minus) operators, will return a datetime.

Get the number of days until QGIS 3.0 release:

to_date('2017-09-29') - to_date(now())
-- Returns <interval: 203 days>

The same with time:

to_datetime('2017-09-29 12:00:00') - to_datetime(now())
-- Returns <interval: 202.49 days>

Get the datetime of 100 days from now:

now() + to_interval('100 days')
-- Returns <datetime: 2017-06-18 01:00:00>

Note

Storing date and datetime and intervals on fields

The ability to store date, time and datetime values directly on fields may depend on the data source’s provider (e.g., shapefiles accept date format, but not datetime or time format). The following are some suggestions to overcame this limitation.

date, Datetime and time can be stored in text type fields after using the to_format() function.

Intervals can be stored in integer or decimal type fields after using one of the date extraction functions (e.g., day() to get the interval expressed in days)

Fields and Values ¶

Contains a list of fields from the layer.

Generally, you can use the various fields, values and functions to construct the calculation expression, or you can just type it into the box.

To display the values of a field, you just click on the appropriate field and choose between Load top 10 unique values and Load all unique values. On the right side, the Field Values list opens with the unique values. At the top of the list, a search box helps filtering the values. To add a value to the expression you are writing, double click its name in the list.

Sample values can also be accessed via right-click. Select the field name from the list, then right-click to access a context menu with options to load sample values from the selected field.

Fields name should be double-quoted in the expression. Values or string should be simple-quoted.

Fuzzy Matching Functions ¶

This group contains functions for fuzzy comparisons between values.

Function	Description
hamming_distance	Returns the number of characters at corresponding positions within the input strings where the characters are different
levensheim	Returns the minimum number of character edits (insertions, deletions or substitutions) required to change one string to another. Measure the similarity between two strings
longest_common_substring	Returns the longest common substring between two strings
soundex	Returns the Soundex representation of a string

General Functions ¶

This group contains general assorted functions.

Function	Description
eval	Evaluates an expression which is passed in a string. Useful to expand dynamic parameters passed as context variables or fields
layer_property	Returns a property of a layer or a value of its metadata. It can be layer name, crs, geometry type, feature count...
var	Returns the value stored within a specified variable. See variable functions below

Geometry Functions ¶

This group contains functions that operate on geometry objects (e.g., length, area).

Function	Description
$area	Returns the area size of the current feature
$geometry	Returns the geometry of the current feature (can be used for processing with other functions)
$length	Returns the length of the current line feature
$perimeter	Returns the perimeter of the current polygon feature
$x	Returns the x coordinate of the current feature
$x_at(n)	Returns the x coordinate of the nth node of the current feature’s geometry
$y	Returns the y coordinate of the current feature
$y_at(n)	Returns the y coordinate of the nth node of the current feature’s geometry
angle_at_vertex	Returns the bisector angle (average angle) to the geometry for a specified vertex on a linestring geometry. Angles are in degrees clockwise from north
area	Returns the area of a geometry polygon feature. Calculations are in the Spatial Reference System of this geometry
azimuth	Returns the north-based azimuth as the angle in radians measured clockwise from the vertical on point_a to point_b
boundary	Returns the closure of the combinatorial boundary of the geometry (ie the topological boundary of the geometry).
bounds	Returns a geometry which represents the bounding box of an input geometry. Calculations are in the Spatial Reference System of this geometry
bounds_height	Returns the height of the bounding box of a geometry. Calculations are in the Spatial Reference System of this geometry
bounds_width	Returns the width of the bounding box of a geometry. Calculations are in the Spatial Reference System of this geometry
buffer	Returns a geometry that represents all points whose distance from this geometry is less than or equal to distance. Calculations are in the Spatial Reference System of this geometry
centroid	Returns the geometric center of a geometry
closest_point	Returns the point on a geometry that is closest to a second geometry
combine	Returns the combination of two geometries
contains(a,b)	Returns 1 (true) if and only if no points of b lie in the exterior of a, and at least one point of the interior of b lies in the interior of a
convex_hull	Returns the convex hull of a geometry (this represents the minimum convex geometry that encloses all geometries within the set)
crosses	Returns 1 (true) if the supplied geometries have some, but not all, interior points in common
difference(a,b)	Returns a geometry that represents that part of geometry a that does not intersect with geometry b
disjoint	Returns 1 (true) if the geometries do not share any space together
distance	Returns the minimum distance (based on Spatial Reference System) between two geometries in projected units
distance_to_vertex	Returns the distance along the geometry to a specified vertex
end_point	Returns the last node from a geometry
exterior_ring	Returns a line string representing the exterior ring of a polygon geometry, or null if the geometry is not a polygon
extrude(geom,x,y)	Returns an extruded version of the input (Multi-) Curve or (Multi-)Linestring geometry with an extension specified by x and y
geom_from_gml	Returns a geometry created from a GML representation of geometry
geom_from_wkt	Returns a geometry created from a well-known text (WKT) representation
geom_to_wkt	Returns the well-known text (WKT) representation of the geometry without SRID metadata
geometry	Returns a feature’s geometry
geometry_n	Returns the nth geometry from a geometry collection, or null if the input geometry is not a collection
interior_ring_n	Returns the geometry of the nth interior ring from a polygon geometry, or null if the geometry is not a polygon
intersection	Returns a geometry that represents the shared portion of two geometries
intersects	Tests whether a geometry intersects another. Returns 1 (true) if the geometries spatially intersect (share any portion of space) and 0 if they don’t
intersects_bbox	Tests whether a geometry’s bounding box overlaps another geometry’s bounding box. Returns 1 (true) if the geometries spatially intersect (share any portion of space) their bounding box, or 0 if they don’t
is_closed	Returns true if a line string is closed (start and end points are coincident), false if a line string is not closed, or null if the geometry is not a line string
length	Returns length of a line geometry feature (or length of a string)
line_interpolate_angle	Returns the angle parallel to the geometry at a specified distance along a linestring geometry. Angles are in degrees clockwise from north.
line_interpolate_point	Returns the point interpolated by a specified distance along a linestring geometry.
line_locate_point	Returns the distance along a linestring corresponding to the closest position the linestring comes to a specified point geometry.
line_merge	Returns a (Multi-)LineString geometry, where any connected LineStrings from the input geometry have been merged into a single linestring.
m	Returns the m value of a point geometry
make_line	Creates a line geometry from a series of point geometries
make_point(x,y,z,m)	Returns a point geometry from x and y (and optional z or m) values
make_point_m(x,y,m)	Returns a point geometry from x and y coordinates and m values
make_polygon	Creates a polygon geometry from an outer ring and optional series of inner ring geometries
nodes_to_points	Returns a multipoint geometry consisting of every node in the input geometry
num_geometries	Returns the number of geometries in a geometry collection, or null if the input geometry is not a collection
num_interior_rings	Returns the number of interior rings in a polygon or geometry collection, or null if the input geometry is not a polygon or collection
num_points	Returns the number of vertices in a geometry
num_rings	Returns the number of rings (including exterior rings) in a polygon or geometry collection, or null if the input geometry is not a polygon or collection
order_parts	Orders the parts of a MultiGeometry by a given criteria
overlaps	Tests whether a geometry overlaps another. Returns 1 (true) if the geometries share space, are of the same dimension, but are not completely contained by each other
perimeter	Returns the perimeter of a geometry polygon feature. Calculations are in the Spatial Reference System of this geometry
point_n	Returns a specific node from a geometry
point_on_surface	Returns a point guaranteed to lie on the surface of a geometry
project	Returns a point projected from a start point using a distance and bearing (azimuth) in radians
relate	Tests or returns the Dimensional Extended 9 Intersection Model (DE-9IM) representation of the relationship between two geometries
reverse	Reverses the direction of a line string by reversing the order of its vertices
segments_to_lines	Returns a multi line geometry consisting of a line for every segment in the input geometry
shortest_line	Returns the shortest line joining two geometries. The resultant line will start at geometry 1 and end at geometry 2
start_point	Returns the first node from a geometry
sym_difference	Returns a geometry that represents the portions of two geometries that do not intersect
touches	Tests whether a geometry touches another. Returns 1 (true) if the geometries have at least one point in common, but their interiors do not intersect
transform	Returns the geometry transformed from the source CRS to the destination CRS
translate	Returns a translated version of a geometry. Calculations are in the Spatial Reference System of this geometry
union	Returns a geometry that represents the point set union of the geometries
within (a,b)	Tests whether a geometry is within another. Returns 1 (true) if geometry a is completely inside geometry b
x	Returns the x coordinate of a point geometry, or the x coordinate of the centroid for a non-point geometry
x_min	Returns the minimum x coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
x_max	Returns the maximum x coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
y	Returns the y coordinate of a point geometry, or the y coordinate of the centroid for a non-point geometry
y_min	Returns the minimum y coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
y_max	Returns the maximum y coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
z	Returns the z coordinate of a point geometry

Some examples:

You can manipulate the current geometry with the variable $geometry to create a buffer or get the point on surface:
```
buffer( $geometry, 10 )
point_on_surface( $geometry )
```
Return the x coordinate of the current feature’s centroid:
```
x( $geometry )
```

Send back a value according to feature’s area:

CASE WHEN $area > 10 000 THEN 'Larger' ELSE 'Smaller' END

Record Functions ¶

This group contains functions that operate on record identifiers.

Function	Description
$currentfeature	Returns the current feature being evaluated. This can be used with the ‘attribute’ function to evaluate attribute values from the current feature.
$id	Returns the feature id of the current row
$map	Returns the id of the current map item if the map is being drawn in a composition, or “canvas” if the map is being drawn within the main QGIS window
$rownum	Returns the number of the current row
$scale	Returns the current scale of the map canvas
attribute	Returns the value of a specified attribute from a feature
get_feature	Returns the first feature of a layer matching a given attribute value
uuid	Generates a Universally Unique Identifier (UUID) for each row. Each UUID is 38 characters long.

Some examples:

Return the first feature in layer “LayerA” whose field “id” has the same value as the field “name” of the current feature (a kind of jointure):
```
get_feature( 'layerA', 'id', attribute( $currentfeature, 'name') )
```

Calculate the area of the joined feature from the previous example:

area( geometry( get_feature( 'layerA', 'id', attribute( $currentfeature, 'name') ) ) )

String Functions ¶

This group contains functions that operate on strings (e.g., that replace, convert to upper case).

Function	Description
char	Returns the character associated with a unicode code
concat	Concatenates several strings to one
format	Formats a string using supplied arguments
format_date	Formats a date type or string into a custom string format
format_number	Returns a number formatted with the locale separator for thousands (also truncates the number to the number of supplied places)
left(string, n)	Returns a substring that contains the n leftmost characters of the string
length	Returns length of a string (or length of a line geometry feature)
lower	converts a string to lower case
lpad	Returns a string with supplied width padded using the fill character
regexp_replace	Returns a string with the supplied regular expression replaced
regexp_substr	Returns the portion of a string which matches a supplied regular expression
replace	Returns a string with the supplied string replaced
right(string, n)	Returns a substring that contains the n rightmost characters of the string
rpad	Returns a string with supplied width padded using the fill character
strpos	Returns the index of a regular expression in a string
substr	Returns a part of a string
title	Converts all words of a string to title case (all words lower case with leading capital letter)
trim	Removes all leading and trailing white space (spaces, tabs, etc.) from a string
upper	Converts string a to upper case
wordwrap	Returns a string wrapped to a maximum/ minimum number of characters

Recent Functions ¶

This group contains recently used functions. Any expression used in the Expression dialog is added to the list, sorted from the more recent to the less one. This helps to quickly retrieve any previous expression.

Variables Functions ¶

This group contains dynamic variables related to the application, the project file and other settings. It means that some functions may not be available according to the context:

from the ^{Select by expression} dialog
from the ^{Field calculator} dialog
from the layer properties dialog
from the print composer

To use these functions in an expression, they should be preceded by @ character (e.g, @row_number). Are concerned:

Function	Description
atlas_feature	Returns the current atlas feature (as feature object)
atlas_featureid	Returns the current atlas feature ID
atlas_featurenumber	Returns the number of pages in composition
atlas_filename	Returns the current atlas file name
atlas_geometry	Returns the current atlas feature geometry
atlas_pagename	Returns the current atlas page name
atlas_totalfeatures	Returns the total number of features in atlas
grid_axis	Returns the current grid annotation axis (eg, ‘x’ for longitude, ‘y’ for latitude)
grid_number	Returns the current grid annotation value
item_id	Returns the composer item user ID (not necessarily unique)
item_uuid	Returns the composer item unique ID
layer_id	Returns the ID of current layer
layer_name	Returns the name of current layer
layout_dpi	Returns the composition resolution (DPI)
layout_numpages	Returns the number of pages in the composition
layout_pageheight	Returns the composition height in mm
layout_pagewidth	Returns the composition width in mm
map_extent_center	Returns the point feature at the center of the map
map_extent_height	Returns the current height of the map
map_extent_width	Returns the current width of the map
map_id	Returns the ID of current map destination. This will be ‘canvas’ for canvas renders, and the item ID for composer map renders
map_rotation	Returns the current rotation of the map
map_scale	Returns the current scale of the map
project_filename	Returns the filename of current project
project_folder	Returns the folder for current project
project_path	Returns the full path (including file name) of current project
project_title	Returns the title of current project
qgis_os_name	Returns the current Operating system name, eg ‘windows’, ‘linux’ or ‘osx’
qgis_platform	Returns QGIS platform, eg ‘desktop’ or ‘server’
qgis_release_name	Returns current QGIS release name
qgis_version	Returns current QGIS version string
qgis_version_no	Returns current QGIS version number
symbol_angle	Returns the angle of the symbol used to render the feature (valid for marker symbols only)
symbol_color	Returns the color of the symbol used to render the feature
user_account_name	Returns the current user’s operating system account name
user_full_name	Returns the current user’s operating system user name
row_number	Stores the number of the current row
value	Returns the current value

Function Editor ¶

With the Function Editor, you are able to define your own Python custom functions in a comfortable way.

The Function Editor tab

The function editor will create new Python files in .qgis2\python\expressions folder and will auto load all functions defined when starting QGIS. Be aware that new functions are only saved in the expressions folder and not in the project file. If you have a project that uses one of your custom functions you will need to also share the .py file in the expressions folder.

Here’s a short example on how to create your own functions:

@qgsfunction(args="auto", group='Custom')
def myfunc(value1, value2, feature, parent):
    pass

The short example creates a function myfunc that will give you a function with two values. When using the args='auto' function argument the number of function arguments required will be calculated by the number of arguments the function has been defined with in Python (minus 2 - feature, and parent).

This function then can be used with the following expression:

myfunc('test1', 'test2')

Your function will be implemented in the Custom functions group of the Expression tab after using the Run Script button.

Further information about creating Python code can be found in the PyQGIS Developer Cookbook.

The function editor is not only limited to working with the field calculator, it can be found whenever you work with expressions.