`format_for` modifies input BytesIO position

[benoit74]

When calling format_for, the input BytesIO position is modified. Not sure this is really expected behavior, at least it caused openzim/sotoki#410 ;

@rgaudin any views on which convention we should have on that aspect?

One example:

svg_content = """<svg data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 60 60">
<rect width="100%" height="100%" fill="#001489"/>
</svg>
"""

svg_bytes = io.BytesIO()
svg_bytes.write(svg_content.encode())
print(svg_bytes.tell())
print(format_for(svg_bytes, from_suffix=False))
print(svg_bytes.tell())

Output:

> python test.py
138
SVG
18

Another example:

svg_content = """<svg data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 60 60">
<rect width="100%" height="100%" fill="#001489"/>
</svg>
"""

svg_bytes = io.BytesIO()
svg_bytes.write(svg_content.encode())
svg_bytes.seek(0)
print(svg_bytes.tell())
print(format_for(svg_bytes, from_suffix=False))
print(svg_bytes.tell())

Output:

> python test.py
o
SVG
18

[rgaudin]

• Write into the buffer is outside of the API but in our context of saving (never appending), that operation should be followed with a seek(0)
• format_for() API is clearly an information retrieving one and should not alter the buffer (well temporarily is OK). To me it's a bug that should be fixed.