openpyxl - adjust column width size

With openpyxl 3.0.3 the best way to modify the columns is with the DimensionHolder object, which is a dictionary that maps each column to a ColumnDimension object. ColumnDimension can get parameters as bestFit, auto_size (which is an alias of bestFit) and width. Personally, auto_size doesn't work as expected and I had to use width and figured out that the best width for the column is len(cell_value) * 1.23.

To get the value of each cell it's necessary to iterate over each one, but I personally didn't use it because in my project I just had to write worksheets, so I got the longest string in each column directly on my data.

The example below just shows how to modify the column dimensions:

import openpyxl
from openpyxl.worksheet.dimensions import ColumnDimension, DimensionHolder
from openpyxl.utils import get_column_letter

wb = openpyxl.load_workbook("Example.xslx")
ws = wb["Sheet1"]

dim_holder = DimensionHolder(worksheet=ws)

for col in range(ws.min_column, ws.max_column + 1):
    dim_holder[get_column_letter(col)] = ColumnDimension(ws, min=col, max=col, width=20)

ws.column_dimensions = dim_holder

You could estimate (or use a mono width font) to achieve this. Let's assume data is a nested array like

[['a1','a2'],['b1','b2']]

We can get the max characters in each column. Then set the width to that. Width is exactly the width of a monospace font (if not changing other styles at least). Even if you use a variable width font it is a decent estimation. This will not work with formulas.

from openpyxl.utils import get_column_letter

column_widths = []
for row in data:
    for i, cell in enumerate(row):
        if len(column_widths) > i:
            if len(cell) > column_widths[i]:
                column_widths[i] = len(cell)
        else:
            column_widths += [len(cell)]
    
for i, column_width in enumerate(column_widths,1):  # ,1 to start at 1
    worksheet.column_dimensions[get_column_letter(i)].width = column_width

A bit of a hack but your reports will be more readable.

My variation of Bufke's answer. Avoids a bit of branching with the array and ignores empty cells / columns.

Now fixed for non-string cell values.

ws = your current worksheet
dims = {}
for row in ws.rows:
    for cell in row:
        if cell.value:
            dims[cell.column] = max((dims.get(cell.column, 0), len(str(cell.value))))    
for col, value in dims.items():
    ws.column_dimensions[col].width = value

As of openpyxl version 3.0.3 you need to use

 dims[cell.column_letter] = max((dims.get(cell.column_letter, 0), len(str(cell.value))))

as the openpyxl library will raise a TypeError if you pass column_dimensions a number instead of a column letter, everything else can stay the same.

Even more pythonic way to set the width of all columns that works at least in openpyxl version 2.4.0:

for column_cells in worksheet.columns:
    length = max(len(as_text(cell.value)) for cell in column_cells)
    worksheet.column_dimensions[column_cells[0].column].width = length

The as_text function should be something that converts the value to a proper length string, like for Python 3:

def as_text(value):
    if value is None:
        return ""
    return str(value)